cement composites
TRANSCRIPT
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 1/220
NATURAL FIBRE REINFORCED
CEMENT COMPOSITES
thesis submitted
for the degree
o
DO TOR OF PHILOSOPHY
by
YONG
N
• cc L I B R R Y y
DEPARTMENT OF MECHANICAL ENGINEERING
VICTORIA UNIVERSITY OF TECHNOLO GY
AUSTRALIA
July 1995
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 2/220
FTS THESIS
620.137 NI
3 1 4466811
N1 Yong
Natural fibre reinforced
cement composites
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 3/220
Acknowledgements
I wish to thank my principal supervisor, senior lecturer Dr. B. Tobias for his interest,
help
and continual en couragement.
I thank my colleagues and friends in Division of Forest Products, Commonwealth
Scientific & Industrial Research Organisation (CSIRO), where I carried out this work.
A special thanks to the staff of the Pulp and Paper Group and the Fibre C omp osites
Group, who assisted me in my transition from student to professional researcher.
They are too many to be listed here. In particular, I wish to thank Mr. N. G. Langfors,
A. W. McKenzie (deceased), D. Menz, N. B. Clark, W. J. Chin and C. Garland, M s.
M. McKenzie and Ms. S. G. Grover, Drs. R. Evans and V. Balodis for their assistance,
advice and criticism at various stages of this investigation.
My debt of thanks is due to Chief Research Scientist Dr. R. S. P. Goutts, who acted as
my external supervisor at CSIRO . I am further indebted to Bob in that he first
introduced me to the field of natural fibre reinforced cement composites / pulp and
paper science, and patiently nurse d me through the techniques of these fields and
fully guided me throughout the course of this work.
Finally, I am deeply indebted to my wife, family members and friends, who have been
continuaUy understanding, encouraging and supportive during the period of this
research project.
I really wish to say thank you
all .
Yong Ni (Philip)
December 1994
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 4/220
N A TU RA L FIBRE REIN FO RCED CEM EN T C O M PO SITES
CONTENTS
Abstract vi
st of Tables viii
List of Figures x
Chapter One: Introduction 1
1.1 Natural fibre reinforced cement composites (NFRC) 1
1.1.1. History and development of NFR C composites 1
1.1.2 Fabrication process 4
1.1.3 Property requirements for asbestos alternatives 7
1.1.4 Mechanical and physical properties of NFR C 8
1.1.5 Durability of NFRC 15
1.2 Natural plant fibre resou rces 16
1.3 Scopes of the present work 19
1.3.1 Bamboo fibre and
bamboo-wood
hybrid fibre reinforced cement
composites 19
1.3.2 Influence of fibre properties on com posites performance 20
Chapter Two: Theoretical Principles of Fibre Reinforcement
22
2.1 Strength and toughness 23
2.1.1 M ixture rule for strength 23
2.1.2
TheACKtiieory
25
2.1.3 Basics of fracture mechanics 28
2.1.4 Fibre critical fracture length 31
2.1.5 Fibre aspect ratio 33
2.1.6 Fracture toughness 35
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 5/220
2.2 Bonding and microstructure of NFR C 38
2.2.1 Chemical bonding 39
2.2.2 Mech anical bonding 40
2.2.3 The effect of mo isture on bond ing 43
2.2.4 Microstructure of NFR C composites 44
Chapter Three: The Nature of Plant Fibres 52
3.1 Fibre classification 52
3.2 Structure of natural plant fibres 53
3.3 Chem ical com position and durability 56
3.4 Preparation of natural plant fibres 59
3.4.1 Pulping 59
3.4.2 Refining or beating 64
3.5 Properties of natu ral fibres 66
3.5.1 Physical properties 67
3.5.2 M echanical properties 69
3.5.3 The effect of moisture on fibres 75
Chapter Four: Air-cured Autoclaved Bam boo Fibre Reinforced Cem ent
Composites (BFRC)
77
4.1 Experimental work 78
4.1.1 Materials 78
4.1.2 Fibre modification 78
4.1.3 Fabrication and characterisation 79
4.2 Air-cured bamboo fibre reinforced cement 80
4.2.1 M echanical properties 80
4.2.2 Physical properties 83
4.3 Autoclaved bamboo fibre reinforced cement 85
4.3.1 M echanical properties 85
4.3.2 Physical properties 92
11
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 6/220
4.4 Conclusions 93
Chapter Five: Bam boo W ood Hybrid Fibre Reinforced Cement Composite
Materials (BWFRC)
95
5.1 Experimen tal work 96
5.1.1 Fibre preparation 96
5.1.2 Fabrication and characterisation 97
5.2 Results and Discu ssion 98
5.2.1 Leng th and freeness of blended pulp 98
5.2.2 Air-cured BW FRC composites 99
5.2.3 Autoclaved BW FRC composites 103
5.3 Theo retical predication and the experim ental results 107
5.4 Conc lusions 108
Chapter Six: Influence of Fibre Length on Co mpo site Properties 110
6.1 Experimental work
112
6.1.1 Fibre length fractionation work 112
6.1.2 Fabrication and characterisation 113
6.2 Results and Discussion 113
6.2.1 Fractionation of fibre length 113
6.2.2 Influence of fibre length on com posite mechanical properties
115
6.2.3 Theoretical and experimental conflict in results 121
6.2.4 Influence of fibre length on com posite physical properties 122
6.3 Conclusions 123
Chapter Seven: Influence of Fibre Strength on Com posite Properties 125
7.1 Experimen tal work 126
7.1.1 Fibre strength fractionation 126
7.1.2 Fibre quality evaluation 127
7.1.3 Com posite fabrication and evaluation 127
7.2 Resu lts and Discussion 128
111
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 7/220
7.2.1 Fibre strength variation work 128
7.2.2 Influence of fibre strength on com posite properties 131
7.3 Conc lusions 138
Chapter Eight: Influence of Fibre Lignin Content on Com posite Properties 139
8.1 Expe rimental work 141
8.1.1 Fibre preparation 141
8.1.2 Fibre lignin content and evaluation of other properties 142
8.1.3 Com posite fabrication and characterisation 142
8.2 Results and discussion 142
8.2.1 Fibre lignin content and other properties 142
8.2.2 Influence of lignin content on air-cured com posites 144
8.2.3 Influence of lignin content on autoclaved composites 149
8.3 Conclusions 153
Chapter Nine: Conclusions and Further Work 155
9.1 Conclusions 155
9.2 Recom mendations for Further Work 157
9.2.1 Pulp supply 157
9.2.2 Fibre length popu lation distribution and cross-dimensions (coarseness) 158
9.2.3 Conformability - flexibU ity and collapsib ility 159
9.2.4 The impact of pulp medium consistency treatment on fibre-cement
products 160
9.2.5 Theoretical modeling 161
Appendix A: Fabrication Characterisation M ethods 162
A.l
Pulp
fibre preparation 162
A.1.1
Chem ical pulping (Kraft pulping) 162
A.1.2
Mechanical pulping (TM P, CTM P) 165
A.l.3
Bleaching pulps (Oxygen delignification) 166
A. 1.4 Holocellulose pulp 169
V
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 8/220
A.1.5 Beating 169
A.l.6 Preparation of fibres from dry lap-pulp 171
A.2 Pulp fibre characterisation 172
A.2.1
Lignin conten t 172
A.2.2
Drainability
(Freeness) 173
A.2.3
Fibre lengtii
175
A.2.4 Handsheet preparation 177
A.2.5 Fibre strength 179
A.3 Fabrication com posite materials 181
A.3.1
M aterials 182
A.3.2 Slurry/vacuum dewatering and press techn ique 183
A.3.3 Air curing and autoclaving 184
A.4 Characterisation of comp osite materials 186
Ap pen dixB : Determination of Kraft pulping parameters 188
References
189
Bibliography 202
V
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 9/220
Abstract
The health problems associated with asbestos and its related products necessitated in
finding alternative resource of fibres. Over the last two decades natural fibre (mainly
wood pulp fibre) has emerged as the most acceptable alternative reinforcement for
fibre cement products. The first three chapters of this study describe in some depth
the preparation and properties of natural fibres, the methods of incorporating such
fibres into cements and mortars, the theoretical princ iples of fibre reinforcement, the
properties obtained from these natural fibre (mainly wood pulp fibre) reinforced
cement composites and their applications as commercial products, especially as the
ma in alternatives to asbestos reinforced cem ent materials. Chapter four and five
discuss fabrication and performance characterisation of the resulting composites.
Whereas, chapters six, seven, eight and nine incorporate results and conclusions. The
detailed experimental procedures and methods are described in Appendix A and B.
Bamboo pulp fibre was investigated as reinforcement for incorporation into cements
and mo rtars. The results show that bamboo fibre is a satisfactory fibre for
incorporation into a cement matrix. The composites so formed have acceptable
flexural strength, but lack fracture toughness due to the short fibre length and the high
fines content of the bamboo
pulp used in this study. Experimentation was con ducted
in an attempt to improve the fracture toughness prope rties of bamboo fibre reinforced
cement com posites. Blending bamboo fibre with varying proportions of softwood
fibre led to a range of materials with improved performance, especially with respect to
the property of fracture toughness.
V
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 10/220
There is a need to be able to specify the properties of natural ceUulose fibres, in
particular wood pulp fibres, when they are to be used as reinforcement in fibre cement
products. Preliminary studies have attempted to isolate specific fibre properties such
as fibre length, fibre strength and fibre lignin conten t. Then the se parame ters w ere
used to relate the mechanical performance of fibre cem ent composites. This study has
shown that the fibre length plays a major role in the developm ent of flexural strength
and fracture toughness of a fibre reinforced composite. As the length increases both
properties improve over a range of fibre con tents. There is som e concern how ever,
over the best manner of specifying this parameter - average fibre lengths or fibre
length distributions. Fibre strength was found to have less effect on com posite
flexural strength than on fracture toughness values, for both air-cured and autoclaved
products. It was somewhat unexpected that weak fibres could provide relatively
strong, but; as might be expected, brittle materials. Fibre lignin content was show n to
have a considerable effect upon the formation of autoclaved products . The re is
difficulty, however, to examine fibre lignin content as an isolated pa rame ter, due to
interaction from fibre strength, stiffness and fibre-matrix interface bonding.
Overall, this study has provided better understanding of the complex behaviour of
natural fibre reinforced cement composites.
vu
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 11/220
List of Tables
Table 1.1 Com parison of properties of fibres for possible asbestos 8
alternatives
Table 1.2 M echanical and physical properties of com mercial W FRC 14
materials based on James Hardie's products
Table 1.3 M echanical and physical properties of laboratory fabricated 14
NFRC
Table 1.4 Annual collectable yields of various non-w ood plant fibrous
18
raw materials
Table 1.5 Availability of various non-wood plant fibrous raw ma terials, 18
1982
Table 1.6 Total production of various non-w ood plant fibre pulps in 1982 19
Table 2.1 Effect of aspect ratio and fibre con tent
4
Table
3.1 Chem ical compositions of natural plant fibres
Table 3.2 Typical softwood and hardwood fibres physical dimensions
Table 3.3 Some non-wood fibres physical dimen sions
Table 3.4 Some grasses pulps physical dimensions
Table 3.5
Stracture
and strength parameters of non-wood fibres
57
67
68
68
72
Table 4.1 Properties of air-cured bamboo-fibre-reinforced cemen t 81
Table 4.2 Properties of autoclaved bamboo-fibre-reinforced cement 86
Table 4.3 Fibre weighted average length (mm) 87
Table 4.4 Fibre length mass distribution percentage 87
Table 4.5 Properties of autoclaved screened long bamboo fibre reinforced 88
cement
Table 5.1 Length and freeness of blended pulp
Table 5.2 Properties of air-cured BW FRC
Table 5.3 Properties of autoclaved BW FRC
99
99
104
vui
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 12/220
Table 5.4 Regression analysis results
1 8
Table 6.1 Fibre length fractions
Table 6.2 Relationship between fibre length and air-cured comp osite
performance
114
115
Table 7.1 Properties of P.radiata fibre after alkaline cooking
Table 7.2 Composites properties reinforced with alkaline treated
holocellulose fibres
Table 7.3 Composites properties reinforced with acid hydrolysis fibres
129
131
136
Table 8.1 Pulping techniques
Table 8.2 Fibre hgnin conten t, freeness and fibre strength , fibre length
Table 8.3 Influence of fibre lignin content on air-cured WFRC
Table 8.4 Influence of fibre lignin content on autoclaved WFRC
141
142
146
150
Table A. 1 Common bleaching chemicals
Table A.2 Pulp test methods
Table A.3 Natural fibre reinforced cement composites ingredients
168
172
182
ix
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 13/220
List of igures
Fig. 1.1 The Hatschek process 5
Fig.
1.2
Graph of flexural strength
via
fibre content for various WFRC 11
products
Fig.
2.1
Schematic representation of crack travelling through a fibre 23
reinforced matrix
Fig. 2.2 Tensile stress strain curves for fibre reinforced brittle matrices 26
predicted by the ACK theory (full line), and the bending
response calculated from them (broken lines)
Fig. 2.3 Theoretical model applicable to low modulus fibre-reinforced 27
cement composite at flexural failure
Fig. 2.4 Tensile load-extension curves for different failure modes of 33
sisal silvers embedded in cement
Fig. 2.5 Possible coupling mechanism between wood fibre and cement 41
matrix
Fig. 2.6 SEM showing fracture surface of W FRC precond itioned at 46
100 - 105"C for 24h
Fig. 2.7 SEM showing fracture surface of WFRC precond itioned by 47
soaking in water for 48h
Fig. 2.8a SEM showing fracture surface of W FRC precond itioned at 50 47
± 5 % relative humidity and 22 ± 2°C
Fig. 2.8b As (a) but higher magn ification 48
Fig. 2.9 SEM showing cement surface at interface contains dense 49
matrix with some discontinuities
Fig. 2.10 SEM shows fractured fibres with dense ma terial from bulk of 49
matrix up to fibre wall
Fig. 3.1 The structure of wood fibre 54
Fig. 3.2 The structure of bamboo fibre 54
Fig. 3.3 Scanning electron micrograph of a cube of eastern wh ite pine 55
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 14/220
microtomed on three surfaces
Fig. 3.4 The principle of chemical and mechan ical pulping 62
Fig. 3.5 (a, top) Unbeaten fibre of P.radiata compared to (b) 66
externally
fibrillated
fibre
Fig. 3.6 A
pulp
fibre consists of cellulose
fibrUs
in largely parallel
71
array, embedded in a matrix of hgnin and hemicellulose
Fig. 3.7 The maxim um strength of a pulp fibre is limited by the 71
inherent properties of the cellulose fibril modified by the
spiral angle of the fibril about the fibre axis
Fig. 3.8 Laboratory beating - strength tests on chem ical pulps from 73
various wood and non-wood plant fibres
Fig. 4.1 Effect of fibre content on flexural streng th for air-cured 80
WFRC and BFRC
Fig. 4.2 Effect of fibre conten t on fracture toughness for air-cured 82
WFRC and BFRC
Fig. 4.3 Effect of fibre conten t on water absorp tion for air-cured 84
WFRC and BFRC
Fig. 4.4 Effect of fibre content on density for air-cured W FR C and 85
BFRC
Fig. 4.5 Flexural
stiength
as a function of percent fibre load ing for 87
autoclaved BFRC and WFRC composites
Fig. 4.6 Fracture toughness as a function of percent fibre loading for 90
autoclaved BFRC and WFRC composites
Fig. 4.7 Typical Load / Deflection graph for autoclaved W FRC and 91
BFRC composites
Fig. 4.8 Density as a function of percent fibre loading for autoclaved 92
BFRC composites
Fig. 4.9 The relationship between density and water absorption for 93
autoclaved BFRC composite
Fig. 5.1 Relationship between pine fibre proportion and furnish pulp 97
X
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 15/220
length weighted average
Fig. 5.2 Relationship between pine fibre proportion and furnish
pulp
98
freeness value
Fig. 5.3 Influence of long fibre (pine) proportion on the air-cured 100
composites flexural strength
Fig. 5.4 Influence of long fibre (pine) proportion on the air-cured 101
composites fracture toughness
Fig. 5.5 Influence of fibre length on the air-cured composites flexural 102
strength at total 8% fibre content
Fig. 5.6 Influence of fibre length on the air-cured composites fracture 102
toughness at total 8% fibre content
Fig. 5.7 Influence of long fibre (pine) proportion on the autoclaved
105
composites flexural strength
Fig. 5.8 Influence of fibre length on the autoclaved com posites flexural 105
strength at total 8% fibre content
Fig. 5.9 Influence of long fibre (pine) proportion on the autoclaved 106
composites fracture toughness
Fig.
5.10
Influence of fibre length on the autoclaved composites fracture
106
toughness at total 8% fibre content
Fig. 6.1 Fibre length popu lation of Bauer-McNett technique four 114
length fraction
Fig. 6.2 Effect of fibre content on com posite flexural strength for 116
different fibre lengths
Fig. 6.3 Influence of fibre length on com posite flexural strength at 8%
117
fibre content
Fig. 6.4 Effect of fibre content on com posite fracture toughness for 118
different fibre lengths
Fig. 6.5 Influence of fibre leng th to fracture toughness at 8% fibre by 118
content
Fig. 6.6 Influence of fibre W L / d on com posite fracture toughness 120
Fig. 6.7 Influence of fibre length on com pos ite density 123
Xll
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 16/220
Fig. 7.1 Relationship between fibre zero-span tensile strength and pulp 129
viscosity
Fig. 7.2 Relationship between fibre strength and fibre length 129
Fig. 7.3 Relationsh ip between fibre strength and pulp freeness 130
Fig. 7.4 Relationsh ip between fibre strength (0-span) and com posite 132
strength
Fig. 7.5 Relationsh ip between fibre strength (viscosity) and com posite 132
strength
Fig. 7.6 Relationsh ip between fibre strength (0-span) and com posite 133
fracture toughness
Fig. 7.7 Relationship between fibre strength (viscosity) and com posite 133
fracture toughness
Fig. 7.8 Fracture surface of com posite reinforced with strong fibre 134
shows fibre pull-out
Fig. 7.9 Fracture surface of com posite reinforced with weak fibre
135
shows fibre fracture
Fig. 7.10 Relationsh ip between fibre acid treated time and com posite 137
strength
Fig. 7.11 Relationsh ip between fibre acid treated time and com posite 137
fracture toughness
Fig.
8.1
Influence of fibre lignin content on air-cured W FRC flexural 145
strength at different fibre content
Fig. 8.2 Influence of fibre lignin content on air-cured W FR C fracture 145
toughness at different fibre content
Fig. 8.3 Influence of fibre lignin content on air-cured W FRC density at 148
different fibre content
Fig. 8.4 Influence of fibre lignin content on autoclaved W FR C flexural 149
strength at different fibre content
Fig. 8.5 Influence of fibre hgn in content on autoclaved W FR C fracture 151
toughness at different fibre content
Xlll
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 17/220
Fig. 8.6 Influence of fibre lignin content on autoclaved W FRC density 153
at different fibre content
Fig. A.
1
Schem atic iUustration of Air-bath and
3-Iitre
pulping vessels
163
Fig. A.2 Open periphery (G) and closed periphery (B) refining plates 167
Fig. A .3 The Valley Niagara beater 170
Fig. A.4 The
PFI
laboratory beater tackle
171
Fig.
A.5
Canada Freeness tester 174
Fig. A.6 The Bauer-M cNett Classifier
/
176
Fig.
A.7
Kajanni FS-200 measurement principle 176
Fig. A. 8 Schematic illustration of Pulm ac Zero-span tester 181
Fig. A.9 Vacuum dewatering casting box 184
Fig. AlO Optimize autoclaving temp erature and time 185
Fig.
Al 1
Optimize composite
OPC:Silica
matrix ratio for autoclaving
xiv
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 18/220
Chap te r One :
Int roduct ion
1.1 Natural Plant Fibre Reinforced Cement Composites (NFRC)
Since the end of the 19th century, asbestos cement has had a wide range of applications,
such as building / cladding sheets, corrugated roofing elements, pipes and tiles.
Because of the well known health risks associated with the use of asbestos fibres, together
with a possible future shortage of asbestos, there has in recent years, been considerable
research into the developm ent of new high-performance reinforcing fibres. A variety of
fibres such as glass, steel, synthetic polymer and natural cellulose fibres have been
evaluated in both laboratory and pilot plant equipment. Am ong these fibres, natural
cellulose fibres (Kraft pulped soft wood fibres) demonstrate both cost effectiveness and
suitable performance to act as a replacem ent for asbestos fibre. James H ardie Industries in
Australia marketed non-asbestos wood fibre reinforced cement (WFRC) boards in 1981
(Anon, 1981).
1.1.1 History and developm ent of NFRC ) composites
Although patents from the last century refer to the use of natural plant fibre as a
component of building materials made from cements and plasters (US Patent,
1884,1899,1900), interest in natural fibre (mainly wood fibre) as a reinforcement for fibre
cement has mainly taken place in the last 10-20 years .
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 19/220
Unfortunately the large fibre cement manufacturing companies are the real custodians of
the history of the fibre cement development and, from the obvious gap in the literature,
they have released very little information about the use of natural fibres in cem ent.
James Hardie and Coy Pty Ltd started manufacturing asbestos cement products in
Australia in 1917 (James Hardie Industries, 1984). James Ha rdie Industries took an active
interest in the use of ceUuIose, as an economic asbestos substitute, in fibre reinforced
cement in the early to mid -1940 s. This work was intensified during the
post-World
War II
years when there was a worldw ide shortage of asbestos fibre. An investigation w as
conducted by Heath and Hackworthy to discover whether paper pulp could be used to
replace asbestos completely or partially in asbestos cement sheets (James Hardie and Co.
Pty Ltd., 1947). Fibres studied included bagasse, groundwood, wheat straw, cement bags
and brown paper. The experimental autoclaved sheets showed brown paper (Kraft) was
the best of the pulp sources, giving greatest strength to the composite material. However,
when asbestos supply was reinstated, this work was discontinued.
Renewed interest in wood fibres began alm ost inadverten tly in 1960. In those day s, the
asbestos fibre board, containing 15% asbestos, was ma de between steel interleaves. James
Hardie's was beheved to be the only group in the world which at that time was steam-
curing its sheets. To make a cheap board as an alternative interleaf, boards were made up
with half the asbestos replaced by wood fibres. This board becam e the first generation
Hardiflex , and full production started in 1964. From the 1960s onwards their products
have contained no more than 8% asbestos, which was about
half
the am ount used by the
rest of the industry.
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 20/220
Attempts to further reduce the asbestos content by adding more wood fibre were
unsuccessful due to the ineffectiveness of these fibres, compared to asbestos, in trapping
the cement particles during formation of the sheet.
James Hardie entered into collaborative research with CSIRO in 1978 to study, among
other things the refining of cellulose fibres in an attempt to overcom e the difficulties of
retaining the cement in the wood fibre reinforced cement sheet (Anon, 1981). By M ay
1981
the new generation of asbestos-free cemen t products - Hardiflex II - was being
comm ercially manufactured. This autoclaved product was totally reinforced by refined
Kraft wood fibres (Goutts, 1982a; Au s Patent, 1981).
In Europe during 1975 - 77, Cape Industries had made boards reinforced with 5% cellulose
and high levels of mineral fiUers called Supalux , Monolux and Verm iculux , mainly
for fire-resistant use. M asterboard and Masterclad were more dense and stronger and
used for external cladding (Harper,
1982).
In 1976 Sweden banned asbestos cement
products, and all asbestos cemen t production was closed down. Other Scandinav ian
countries were forced to use alternatives, and A /S Norcem in Norway and OY Partek AB
in Finland decided to form a joint development under the name of NOPA (Pedersen,
1980). The material produced was called Cellcem , and contained cellulose fibres mixed
with other fibres. Manufacture of the products Intemit and Pemit started in Norw ay in
1977.
In 1979 Finland started to produce Minerit .
In was stated in 1985 that the UK m anufacturers had replaced asbestos in about 50 % of the
fibre cement sheeting products (Crabtree, 1986). James Hardie Industries by this time had
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 21/220
totally replaced asbestos fibre from its range of building products, which included flat
sheet, corrugated roofing and moulded products.
As well as flat sheet products, James Hardie Industries had become a world leader in
injection moulded fibre cement products and non-pressure fibre cement pipes, all based on
wood fibre as the reinforcement material. The first experimental production of W FRC
pipe was undertaken at the Brooklyn factory in September 1980. Com mercial production
began in Westem Australia at the Welshpool factory in July 1984. The last asbestos pipes
made by James Hardie were manufactured in March 1987.
At the present time there is considerable activity in the paten t literature concerning the use
of natural plant fibres (mainly wood fibres) or mixtures of natural plant fibres (mainly
wood fibres) w ith other synthetic fibres. This is taking place through out Europe and
Japan, and companies such as Dansk Eternit-Fabrik A
/S ,
Cape
Boards
and Panels Ltd,
Partek of Finland, Asano of Japan and others are involved.
1.1.2 Fabrication processes
The manufacture of asbestos fibre cem ent products is a ma ture industry. Hodgson (1985)
suggested that over 1100 sheet machines and 500 p ipe machines were in production
around the globe, with total capital investment in excess of A$ 4.6 billion. The Hatschek
process (or wet process) is the most w idely used method of production (see Figure 1.1).
The manufacturmg techniques are closely related to conventional heavy paper and board
making processes . An aqueous slurry of asbestos and cement matrix, about 7-10% solids
by weight, is supplied to a holding tank which contains a number of rotating screen
cylinders. The cylinders pick up the solid matter removing som e of the water m the
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 22/220
operation. An end less felt band travels over the top surfaces of the cylinders and picks up
a thin layer of formulation from each cylinder. The built-up laminated ply then travels
over vacuum de-watering devices which remove most of the water. The formulation is
then wound up on a steel calender, or assimilation roll, untU a product of desire thickness
is formed. The ma terial is further compressed by pressure rolls which are in contact with
the assimilation rolls.
SLUR YFEEO
C LENDER
SCHEEN CYLINDERS
P OOUCT
H TSCHEK PROCESS
Fig. 1.1. The Hatschek process.
For sheet production the layer built up on the assimilation roll is automatically cut off and
drops onto a conveyor to be transferred to a stack for curing. If corrugated roofing is to be
made, the flat sheet is taken off to a corrugating station where the sheets are deposited
onto oiled steel moulds for shaping. Pipe mach ines are similar to the Hatschek p rocess but
usually have only one or two vats in series. The pressure imposed on the mand rel by the
press roUs is much greater than for sheet production so as to form a dense prod uct. The
machine may be stopped whUe the mandrel carrying the pipe is set to one side for p ipe
withdraw al. The process is often referred to as the Mazza process.
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 23/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 24/220
more traditional air-cured products require 14-28 days of air-curing before they can be
dispatched, this involves considerable stock inventory. The air-curing process is lower in
capital outlay, as no high pressure autoclaves and steam raising plant are required;
however, cement is more expensive than silica, and therefore material costs are higher.
1 1 3 Property requirements for asbestos alternatives
After reviewing generally the processes used to manufacture asbestos fibre cement, a
number of requirements of replacement fibres can be noted, if the existing capital
intensive equipment is to be used. For the Hatschek p rocess a replacement fibre must be
water dispersable in a relatively dilute slurry and able to form a film on the screens. At the
same time the fibre must be
able
to resist chem ical attack due to the high alkalinity (~ pH
13) of the matrix. If the product is to be autoclaved, resistance to temperatures above 170
°C is also required. The basic essentials of cost, availability and mechanical performance
of the fibre are obvious.
As has been reported glass, steel, carbon, synthetic organic as well as natural plant fibres
(mainly wood pulp fibre) have been under examination for use in cement systems. We will
look at a comparison of the properties of these fibres as possible asbestos replacem ents in
existing processes in Table 1.1.
For countries committed to autoclaved products the combination of high alkalinity and
high temperature eliminates most fibres apart from natural plant fibres (mainly wood pulp
fibre), steel, carbon and aramid fibres. The cost of the latter two is almost a factor of
twenty times higher than natural plant fibres (eg. wood pu lp fibre) and so look
unattractive. Steel fibres have processing problem s. If one considers air-curing fibre
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 25/220
cement, to ehm inate the temperature prob lem s, there are still processing Hm itations. The
inorganic fibres such as steel or glass, tend to be too stiff or dense to perform well during
fUm-forming
from dilute slurries;
whUe
the organic fibres lack a surface suitable for
bonding to the matrix and / or introduce drainage problem s. Mixtures of organic fibres
(mainly
PVA)
and natural plant fibres (mainly wood
pulp
fibre) fibres were successfuUy
used to produce air-cured products in Europe (Studinka, 1989).
Table 1.1 Comparison of properties of fibres for possible asbestos alternatives (Goutts,
1988).
Fibre
Wood pulp (chem)
Wood pulp (mech)
Polypropylene
PVA
Kevlar
Steel
Glass
Mineral fibre
Carbon
Alkalia. resist.
1
2
1
1
1
1
3
3
1
Temp
resist.
1
2
3
3
Process ability
1
2
3
3
2
3
3
3
3
Strength
1
2
3
1
1
3
3
3
1
Toughness
1
3
3
1
1
3
3
3
1
Price
3
3
2
2
1
2
2
3
1
1.
High, 2. Medium, 3. Low.
1 1 4 M echanical and physical properties of N FRC
As stated above, natural plant fibre (mainly w ood fibre) is the most cost / performance
asbestos alternative. Natural plant fibre contains cellulose, hemicellulose and hgn in. The
cellulose fibre is the main reinforcing elem ents. M ost natural plant fibres con tain more
than 45% cellulose and some fibres even yield great than 75% cellulose (see section 3.3).
Natural fibre has been successfully employed as an asbestos fibre alternative either by
itself or as mixture with other synthetic fibre for 10 - 15 years . James Hardie Industries in
Australia manufactured a varies of autoclaved fibre cement sheets and pipes reinforced
with about 8%-10% beaten softwood P. radiata) Kraft pulp. The Etemit Group in
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 26/220
Switzerland promoted various types of cellulose fibre reinforced autoclave composites in
South Africa and some air-cured cement products in Europe and Latin America reinforced
with synthetic fibres and natural c ellulose fibres.
The amount of data available on natural fibre reinforced cement (NFRC) products, in the
scientific literatures, has been limited d ue to the fact that manufacturing interests had been
responsible for much of the preliminary work and for commercial reasons had retained the
knowledge in house or locked away in patent literature. Unfortunately, due to the
difficulty in handing theoretical treatments involving natural fibres, there was
less
interest
from the academic fraternity than in say cem ent materials containing glass, steel or
synthetic organic fibres which form the basis of a voluminous scientific literature.
This chapter will only address som e selected resu lts, relating to products containing
natural plant fibres (mainly wood fibres) as the sole reinforcement for cem ent matrices, in
order to give an appreciation of various effects.
1.1.4.1
Strength and fracture toughness of NFRC
During last two decades a number of papers have appeared in which wood fibres are the
sole source of fibre reinforcemen t. These studies have included chemica l and mechanical
pulps of softwoods and hardwoods in air-cured and autoclaved matiices (Goutts,
1985,1986,1987a).
It will be seen that refined wood fibres can afford a strong, tough and durab le fibre
cement, when produced commercially by traditional slurry/dewatered systems followed by
autoclaving. Such WFRC formulations can be used for the production of flat
sheeting.
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 27/220
corrugated roofing, moulded products and low-pressure pipes which traditionally have
used asbestos fibre. Sometimes the laboratory experiments are misleading with respect to
the manufacturing processes and care must be taken in extrapolating the laboratory results
into production.
Although natural cellulose fibre (eg. wood pulp fibre) is cheap and readily processed it has
the disadvantage of being hygroscopic. The composite properties are altered by absorption
of water and, for this reason, extensive testing when bo th wet and dry is required. W FRC
products are generally loaded in bending and so flexural strength has more meaning than
tensile strength in the characterisation of these m aterials.
At CSIRO A ustraha, there was an interest in using high yield pulps [(thermomechan ical
pulp (TMP) and chemithermomechanical pulp (CTMP)], as an alternative to chemical
pulps,
for reinforcing fibre cements. Such pulps make less demand on the forest resources
for a given quantity of pulp (yields twice that of chemical pulp s), less problems with
effluent treatment, chemical requirements are much lower and processing plants are
economical at a smaller scale. Coutts (1986) reported that mechanical pulps
of P radiata
in general were unacceptable as cement reinforcement when autoclaved (with MOR less
than the matrix) but when air-cured had flexural strengths greater than 18 MPa at 8-10%
by mass of fibre. This compares poorly when one notes flexural strengths of WFR C's
containing chemical pulps of P
radiata
are in excess of 20 MPa when autoclaved and 30
MPa when air-cured. W hen autoclaving mechanical pulps the high temperature and
alkalinity virtually chemically pulps the fibres releasing extractives of polysaccharides
and wood acids, which poison the matrix near the fibre causing poor interfacial bonds.
1
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 28/220
Air-curing is less drastic with respect to chem ical attack, hence better properties are
evident in the final composites as shown in Figure 1.2.
30
^
X
w
ui lO
Q KR F T AlR-CUmai
KR F T UT O CL V E D
O
TM P
AIR-CUREO
TMP ijrOCL VED
e 8 10 12
RBRE COKTENT ( BY MASS
Fig, 1.2. Graph of
flexural
strength v. fibre content for various WFRC prod ucts (Coutts, 1988).
Although it was documented from work in the laboratory that Kraft wood fibre was
effective as a reinforcem ent
in
a cement matrix (Coutts, 1979a), the pulp performed poorly
on a pilot-plant Hatschek machine because the fibres were unable to form a web capable
of retaining cement and silica particles. The open nature
of
the web perm itted rapid
drainage, with
loss of
matrix, resulting
in
low product strength.
A
collaborative project,
between CSIRO and James Hardie Industries, starting in 1978 resulted in laboratory data
which demonstrated the benefit of refining wood fibres for use in W FR C materials
(Coutts, 1982a, 1986; Aus Patent, 1981). The breakthrough that made com mercial
production possible came about from the work
of
the Hardie's team
in
adapting the fibre
refining step
to
suit the Hatschek machm e. Before launching the product in
1981
over
11
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 29/220
50,000 sheets had been prepared and tested on the pilot-plant, and about $10 miUion
invested in installing refining equipm ent in the factories.
The effect of moisture on the strength properties of WFRC composites is of importance,
and early on in the research variations in test conditions pro duced variations in test results.
It was evident that standard conditions must be adopted. The flexural strength of WFRC's
can be reduced to as low as 50% of dry strength values in laboratory samp les, in the case
of commercial products the reduction is considerably less but is taken into consideration
for product application.
Fordos and Tram (1986) reported WFRC's containing micro silica with excellent strength
values ranging between 25-55 MP a. A very high stack compression pres sure,
approximately 20 M Pa, was used. Whereas most results usually report pressures of
approximately 2-3 MPa. Cou tts and Warden (1990a) demonstrated the effect on
compaction on the properties of air-cured WFRC and showed that flexural strength
increased with casting pressu re without resulting in a reduction in fracture toughness. Few
workers have reported the elastic mod ulus of W FRC ma terials. Andonian et.al.(1979)
have shown that both tensile and bending moduli are reduced from approximately 13 GPa
to 9 GPa as the fibre content increases from 2 to 10% fibre by m ass.
Fracture toughness is perhaps the most important property for a building material.
Although strength and stiffness are important, the ability of a ma terial to absorb im pact
during handling can decide whether it will find an application in the market place. Fibre
type, pulping method, refining conditions and test conditions all have an effect on the
fracture toughness results of a given formulation. As the fibre content increases up to
12
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 30/220
about 8-10% by mass the fracture toughness increases rapidly then starts to taper off
Values of fracture toughness in excess of 60 times matrix values can be obtained with 10%
fibre by mass . Fracture toughness increases even further when tested wet. This effect will
be discussed when we consider bonding and microstructure. TM P pulps when com pared
with chemical pulps of the same species tend to have fracture toughness values less than
half
that of the chemical pulp reinforced com posite. This can be partly explained in terms
of fibre number and fibre morphology (Goutts, 1986). The variation of fracture toughness
values between softwoods and hardwoods can be attributed to fibre length and fibre
morphology, which we will see is so important for fibre pull-out which takes place during
failure under load.
Beside softwood fibre, considerable research has been conducted on some hardwood
fibres,
non-wood natural plant fibres and waste paper in order to search for cheaper and
more naturally available fibre resources and to better understand the natural fibre
reinforced cem ent based composite ma terials. The mechanical and physical properties of
commercial WFRC and some laboratory fabricated NFRC are listed in Table 1.2 and
Table 1.3, respectively.
It can be seen from Table 1.2 and 1.3 , when natural fibre are prepared by the chemical
pulping method, they can produce acceptable fibre cement products, when either air-cured
or autoclaved, with flexural strength properties comparable to those of softwood.
However, their fracture toughness values tend to vary and most of them less
than WF RC.
Such phenomena might be explained by different fibre parameters such as fibre length {eg.
hardwood (Coutts, 1987a), bamboo (Coutts, 1994a), waste paper (Goutts, 1984a)}, fibre
13
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 31/220
content {eg. TMP (Coutts, 1986)}, fibre strength {eg. NZflax (Coutts, 1983c, 1994b)} and
helical angle {eg. banana (Coutts, 1990b)}, w hile the other phenom ena remain uncertain.
Table 1.2 Mechanical and physical properties of commercial WFRC materials based on
James Hardie's products (Coutts, 1988).
Product
Villaboard II
(6 mm)
Hardiflex II
Compressed
sheet II
(12 mm)
He x. Strength (MPa)
RH
19.2(L)
13.5Cr)
16.3(av)
24(L)
12(T)
18(av)
29.4(L)
22.1(T)
25.7(av)
Wet
12.9(L)
8.6(T)
]0.7(av)
/
/
/
21.0(L)
15.9 (T)
18.4(av)
Mod. of Blast. (GPa)
RH
9.2(L)
8.5(T)
8.9(av)
/
/
/
16.6(L)
15.3(T)
15.9(av)
Wet
6.1(L)
5.2(T)
5.7(av)
/
/
/
12.7(L)
12.3(T)
12.5(av)
Frac. Tou
RH
6.7(L)
3.0(1)
4.9(av)
/
/
/
4.2(L)
2.4(T)
3.3(av)
ghnessCkJ/m' )
Wet
9.0(L)
5.6(1)
7.3(av)
/
/
/
19.5(L)
8.I(T)
13.8(av)
Density
(g/cm^)
1.31
"
1.40
"
1.62
"
W.Abs.
(%)
32.1
"
"
30.5
"
18.6
"
*L: longitudinal direction of manufacture
*T:
transverse direction
Table 1.3 Mechanical and physical properties of laboratory fabricated NFRC
Pulp Fibre
Curing Strength(MPa)
Toughness (kJ/m ) Density(g/cmO W.Abs.(%)
Reference
Kraft
"
II
tt
II
tt
"
"
"
ti
TMP
softwood
"
hardwood
abaca
NZflax
banana
ti
sisal
bamboo
"
softwood
air
auto
air
air
auto
air
auto
auto
air
auto
air
auto
30.3
23.1
20.3
27.3
23.2
20.0
18.5
18.3
17.0
15.5
12.3
9.5
1.93
1.86
1.37
2.08
0.84
0.83
0.55
2.49
0.34
0.29
0.57
0.89
1.55
1.31
1.45
1.55
1.31
15.4
/
1.37
1.59
1.41
1.47
1.29
21.1
33.9
25.8
21.9
35.0
22.9
/
27.6
16.7
30.7
17.8
30.5
Coutts85
Coutts 84 a
Coutts87a
Coutts87b
Coutts83
Coutts90b
Coutts90b
Coutts92a
Coutts94b
Coutts94a
Coutts86
Coutts86
*A11 composites containing
8% fibre
(by mass)
*TMP
denotes thermomechanical pulp
1.1.4.2
Physical properties of NFRC
The physical properties of NFRC materials can have a considerable influence on their
acceptability for use in the construction industry. If a product is strong and tough and has
low density it wiU be preferred by the workers, who handle such produc ts on the building
4
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 32/220
site,
com pared to similar materials that are dense . At the same time due consideration
must be given to water absorption, for as the density is lowered, the void volume increases
with an associated potential increase in water absorption. Thus a load on a structure may
be considerably increased should the material become wet, with more than 30% increase
in weight occurring in some laboratory cases. High tempe rature mechan ical pulps are very
stiff, compared to chemical or low temperature m echanical pulps, and cause poor packing
as the fibre content increases. As void volum e increases w ith poor packing so the density
decreases and water absorption increases. Ma trix ma terial
also
affects the density and
water absorption. Air-cured NF RC materials are more dense than autoclaved m aterials.
1 1 5 D urability of N FR C
Considerable do ubt has been cast on the ability of natural fibres to resist deterioration in
cement matrices, yet no evidence has been put forward to support the claims. When poor
mechanical performance of the composite has been offered as confirmation of fibre failure,
due consideration should be given to the potential of the fibre strength loss and to
poisoning of the matrix surrounding the fibre, resulting in weak interfacial bonding. An
extensive review by Gram (1983) reflects this uncertainty.
Lola (1986) reported the failure of natural fibre cement products after only a few years of
service. In many cases the reinforcement fibre used w as aggregates of fibres and the high
alkalinity (matrix), coupled with cycles of wetting and drying, pulped the fibre bundles
resulting in loss of fibre strength, poisoned cem ent and weak interfacial bond s, and thus
low durability. On the other hand there are many claims which suggest that natural fibre
remforced cement products are durable after 30 years of service.
15
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 33/220
Sharman and Vautier (1986) have done some excellent work on the durability of
autoclaved WFRC products at the Building Research Association of New Zealand. They
discussed the possible ageing mechanisms of corrosion, carbonation, moisture stressing
and microbiological attack.
Akers and co-workers (1986) have published a series of papers which discuss the ageing
behaviour of cellulose fibres both autoclaved and air-cured, in normal environments and
accelerated conditions. Testing had taken place which showed exposu re of WFR C
composites to natural weathering led to an overall increase in flexural strength and elastic
modulus after 5 yea rs. The same workers found that air-cured W FRC products when
aged, either normally or by accelerated m eans , showed a marked reduction in fracture
toughness. The ageing of autoclaved m aterials did not result in mineralisation of the fibres
and maintained good levels of fracture toughn ess. The need to use synthetic fibres in air-
cured products was apparent, however, with aging there appears to be an increase in the
interfacial bond which leads to a greater occurence of fibre failure rather than pull-out and
thus leads to higher strengths but lower fracture tough ness. A general picture is emerging
as more studies are conducted that the autoclaved WFRC products are more durable than
the air-cured hybrid composites which contain mixtures of cellulose and synthetic organic
fibres.
1 2 Natural Plant Fibre Resources
The use of natural plant fibre as reinforcement in building products has been known since
man made m ud bricks using stiaw or sailed the seas in boats made from reeds and sealed
with resins or bitumen. The main use of such fibres, in developing coun tries, has been to
provide cheap, relatively low performance m aterials. The tendency in developed countries
16
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 34/220
was to neglect the research of natural plant fibres for use in cement composites, that is
until the explosion of interest , as evidenced by the scientific and patent literature, which
occurred in the mid 1980's and is expanding to the present time .
Natural plant fibres exist in large quantities all over the world including wood fibre and
nonwood plant fibre. Great amounts of non-wood natural plant fibre are available and
produced in most developing co untries. For examp le, in India alone, some 6.0 million
hectares of land is occupied with banana plantations and it was stated that 3 milhon tons of
banana fibre are available (Coutts, 1990b). Bamboo is another readily available fibre
source, there are altogether 62 genera and over 1000 species of bamboo in the world, of
which 37 genera and about 700 species grow in Asia. China has the greatest num ber of
bamboo species and the area of bamboo plantation in China is 3.2 million hectares, which
is one fifth of the total bam boo grove coverage in the world (Zhao, 1990).
An important factor in the availabUity of any plant for fibre is its collectable yield per unit
land area. Such estimated yields are given for a number of non-wood plants in Table 1.4,
first as collectable raw m aterial, then as the estimated equivalent in bleached pulp.
Based on the total world-wide production of agricultural crops and the land area planted in
each crop, it is possible to make reasonably accurate estimates of the total amount of each
agricultural residue, useful as fibre, wh ich m ight be collected in each country. Sunilar
estimates can be made for crops grown specifically for their fibre con tent. How ever, for
natural growing species, e.g. reeds and bam boo , such estimates are far more difficult, and
accurate data are not available.
7
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 35/220
Table 1.4 An nua l collectable yields of vario us non- wo od plan t fibrous raw ma terials
(estimated) (Atchison, 1983).
Fibrous raw material Collectable as raw material Equivalent in bleached pulp
BD metric tons per hectare year BD metric tons per hectare year
Sugar cane bagasse 5.0 - 12.4 1.7 - 4.2
Wheat straw 2.2- 3. 0 0.7-1.0
Rice straw 1.4-2.0 0.4-0.6
Barley straw
1.4-1.5
0.4 - 0.5
Oat straw 2.2 -3 .0 0.4-0.5
Rye straw 1.4-2.0 0.8-1.0
Bamboo, natural growth 1.5 - 2.0 0.6 - 0.8
Bamboo, cultivated 2.5 - 5.0 1.0 - 2.1
Reeds in the USSR 5.0 - 9.9 2.0 - 4.0
Kenaf-total
stem weight 7.4- 24.7 3.0 - 9.9
Kenaf bast fibre 1.5 - 6.2 0.7 - 3.2
Crotalaria bast fibre
1.5
- 5.0 0.7 - 2.5
Papyrus in Upper Sudan 20.0 - 24.7 5.9 - 7.4
Abaca (Manila hemp) 0.7 - 1.5 0.4 - 0.7
Seed flax straw
1.0-1.5
0.18-0.27
Cotton staple fibre 0.3 - 0.9 0.25 - 0.86
Corn stalks 5.5 - 7.0 1.55 - 1.95
Sorghum stalks 5.5 - 7.0 1.55 - 1.95
Cotton stalks 1.5
-
2.0 0.60 - 0.80
Table 1.5 presents estimates giving a reasonably good indication of the tremendous
quantities of these non-wood plant fibres which can become available if economic
necessity requires their use as papermaking and reinforcing raw materials.
Table 1.5 Availability of various non-wood plant fibrous raw materials, 1982 (estimated)
(Atchison, 1983)
Raw material Potential world-wide availability BD tons
Straw (wheat, rice, oat, barley,
rye,
seed flax, grass seed) 1,145,000,000
Sugar cane bagasse 75,000,0(X)
Bast
fibres
(jute,
kenaf,
roselle, true hemp) 2,900,000
Core material from jute, kenaf, hemp 8,000,000
Leaf fibres (sisal, abaca, henequen) 480,000
Reeds 30,000,000
Bamboo 30,000,000
Papyrus 5,000,000
Corn stalks and sorghum stalks 900,000,000
Cotton stalks 70,000,000
An estimate of world-wide pulp production from nonwood plants, by type of fibre, is
given in Ta ble 1.6. Th ese figures include prod uctio n for dissolving
pulp .
Th ese fibres are
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 36/220
mainly used in paper and pape rboard prod ucts. H ow eve r, these fibres also provid e ready
reinforcement source for composite materials, such as fibre reinforced cement products.
Table 1.6 Total production of various non-wood plant fibre pulps in 1982 (estimated)
(Atchison, 1983)
Type of no-wood plant pulp World production (air-dry tons)
Cereal straw-mainly wheat and rye
1,390,000
Rice straw 750,000
Bamboo 960,000
Bagasse 1,600,000
Reeds
1,400,000
Cotton
Unters
(paper grade and dissolving pulp) 360,(X)0*
Esparto and sabai grass 120,000
Rags, abaca,
flax,
seed straw,
hemp,
sisal and other plant fibres 1,420,000
* Includes about 60,000 metric tons for paper grade
pulp,
remainder for dissolving pulp & nonwovens.
1.3 Scope of the Pre sen t W o rk
1.3.1 Bamboo fibre and bamboo wood hybrid fibre reinforced cement composites
The fibre cement industry has moved towards autoclaved natural plant fibre (wood)
reinforced cement mortars as the most commercially viable product to replace asbestos
cement produc ts. In those countries without adequate forest resources but which have a
great amount of other natural plant fibre resou rces, then manufacturing fibre cement
products with these
non-wood
plant fibre would be of great advantage.
Bamboo is a rapid grown natural plant, which has good fibre qualities and is widely used
in the paper industry throughout the Asia region. Although bamboo has been used in
various forms in the construction mdustry, there is limited information in the scientific
literature concem ing the use of bamboo pulp fibre. One objective of the current work is to
evaluate bamboo
pulp
fibre reinforced cement composites properties and study bamboo
fibre combined with wood fibre in order to imp rove the composites performance. The
work is covered under the following three topics:
19
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 37/220
1.
Air cured bam boo pulp fibre reinforced cem ent com posites;
2.
Autoclaved bam boo pulp fibre reinforced cem ent com posites;
3.
Hybrid bamboo and wood pulp fibre reinforced cement composites.
1.3.2 Influence of fibre properties on composite performance
Much has been written about the enhancement of engineering properties of cement based
products by the inclusion of fibres as reinforcement. Excellent texts and reviews on the
basic concepts of fibre reinforcement of brittle matrices are available, which deal with
synthetic fibres such as steel, polyprop ylene, glass and more recently kevlar and carbon.
Such fibres are homogeneous with respect to chemical composition and anatomical
structure, they have uniform cross-section area and can be obtained at a constant length,
and their tensile strength and modu lus are constant or could be easily modified.
Unfortunately, natural
pulp
fibres are not homogeneous in chemical composition nor
uniform in shape or size even from one given species. At the same time the tensile
strength and modulus varies from fibre to fibre within a given pulp source.
The development of asbestos free fibre cement industry has made it most desirable to
obtain definite information on the relationship between the nature of the natural plant
fibres and their composites properties. It has been generally recognized that the fibre
length and strength are two of the most important factors but, because various features of
fibre morphology and chemical composition can influence composites properties, it has
been difficult to obtain a clear picture of the effect of any one property.
Further work is aimed at developing a fibre m odel w hich identifies those properties of
wood pulp fibre that are most significant for the production of fibre cement products. This
2
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 38/220
model may assist in identifying altemative cellulose fibre resources suitable for
reinforcement and to better understand natural fibre reinforced cement based composites.
The experiment work undertaking
will
be directed at better understanding of the
following:
1. Influence of fibre length on composite properties;
2. Influence of fibre strength on com posite prop erties;
3.
Influence of fibre lignin content on composite properties.
21
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 39/220
Chapter Two:
Theoretical Principles of Fibre Reinforcement
If the maxim um benefit of composite materials as engineering m aterials is to be achieved,
it is necessary to understand their potential for bearing loads. Failure in a fibre composite
emanates from defects in the material. These may be broken fibres, flaws in the matrix
and debonded fibre / ma trix interfaces. Figu re 2.1 shows a schematic representation of a
cross-section through a fibre reinforced m atrix. The diagram shows several possible local
failure events occurring before fracture of the composite. At som e distance ahead of the
crack, which has started to travel through the section, the fibres are intact. In the high-
stress region near the crack tip, fibres m ay debond from the matrix (eg. fibre 1). This
rupture of chemical bonds at the interface uses up energy from the stressed system.
Sufficient stress may be transferred to a fibre (eg. fibre 2) to enable the fibre to be
ultimately fractured (as in fibre 4). When total debonding occurs, the strain energy in the
debonded length of the fibre is lost to the material and is dissipated as heat. A totally
debonded fibre can then be pulled out from the matrix and considerable energy lost from
the system in the form of frictional energy (eg. fibre 3). It is also poss ible for a fibre to be
left intact as the crack propagates. The process is called crack b ridging.
From this simplistic approach we are immediately made aware of the importance of fibre
to matrix bond strength, frictional stress opposing pull out, tensile strength of the fibre,
fibre length and fibre content.
22
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 40/220
CRACK
Fig. 2.1. Schematic representation of crack travelling through a fibre reinforced matrix
2.1 Stren gth a nd Toughn ess
2.1.1 Mixture rule for strength
The law of mixtures for ahgned continuous fibrous composites may be modified to predict
the strength of natural fibre reinforced cements (M ai, 1978). It will be assumed that at
failure the fibres are not broken bu t are pulled out of the cement ma trix. Thus the stress in
the fibre
7,)
is given by:
7f =2r l/d)
(2.1)
where Tis the fibre-matrix interfacial bond strength, I and
d
are the length and diameter of
the fibre. To account for the random distribution of the discontinuous short fibres,
Romualdi and Mandel (1964) suggest that the effective fibre volume fraction is 41% of the
nominal volum e fraction. It is therefore possible to rewrite the ultimate tensile strength
(cTf) equation for the natural fibre reinforced cement as :
7;=
(T,,, v.,; -I- 0.410}Vf
2.2)
23
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 41/220
Equation (2.2) is reduced to equation (2.3) by substituting equation (2.1) for ov:
<7; = 7r„ v..
+
0.82T
vj l/d) (2.3)
In equation (2.3),
<7„,
is the tensile strength of the un-reinforced cement mortar matrix and
Vr,i,
v/ are the volume fractions of the ma trix and the fibre respectively. Equation (2.3) may
be extended to predict the mo dulus of rupture (o),) of fibre cem ent since in general we
have
Of,
= aa, and Jr,n> = P<ym, where a, P are constants which can be determined from
experiments and
Gmh
is the modulus of rupture of the cement mortar matrix in bending.
Thus,
Jh
=
[a/pj
T,„bV„:
+
0.82 ax}vf l/d} (2.4)
CUT
may be regarded as the fibre-matrix interfacial bond stress in flexure.
Equations (2.3) and (2.4) are first given by Swamy and Mangat (1974) for steel fibre
reinforced concrete. Although they have no t experimentally proven the validity of the
ultimate tensile strength as predicated by equation (2.3) they have however show n that
equation (2.4) is valid for the pred iction of ultima te flexural strength of concrete
reinforced with randomly
distiibuted
short discontinuous steel fibres.
The values of a,
/?,
l/d. Cm, cj„,>, and x are suggested to be 2.96, 2.81, 135, 9.71 MPa, 27.27
MPa and 0.35 ~ 0.45 MPa, respectively. Andon ian and Mai (1979) attempted the first
theoretical analysis of the strength properties of WFRC composite using the mixture rule
for random fibre-cement using equations 2.3 and 2.4. The calculations gave close
predictions for the experimental results they ob tained. W hile the theory predicts a
continuous increase in bending strength with increasing fibre mass fraction, the
experimental results show no strength imp rovem ent beyond 8% fibre mass fraction. This
24
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 42/220
is probably a consequence of the relatively large void fractions at larger fibre mass
fraction, which cause further reductions in the interfacial bond strength and matrix
strength due to poor compaction.
2 1 2 The ACK theory
In the case of fibres reinforced cement and gypsum plaster, how ever, the matrix is brittle
and fails at a strain very much lower than the failure strain of the fibres, and it is widely
accepted that the tensile strength depends on the fibre contribution alone and equation
(2.3) is given simply by:
a;^0.82Tv/l/d)
(2.5)
While there is considerable experimental support for mixture rule predicting the tensile
strength and flexural strength of fibre reinforced brittle matrices although it is difficult to
see a mechanism that would allow a matrix contribution to the tensUe strength once the
matrix has failed.
Aveston
et
al.
(1971) have defined the salient points on the tensile stress / strain curve for
composites such as glass reinforced cement where the fibre is more extensive than the
cement, and there are sufficient fibres to support the extra load when the matrix cracks
(The ACK theory ). Tensile stress /
sU-ain
curves predicted by the ACK model are shown
in Fig. 2.2 (full lines). For an id eal com posite there is an initial elastic response after
which the matrix cracks and continues to crack at constant stress and increasing composite
strain. The multiple cracking process continues until the distance between cracks is too
small to allow transfer of sufficient load from fibre to matrix to crack it further.
Thereafter, further increase in load is taken by the fibres alone, and they extend and slip
25
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 43/220
relative to the matrix until they break or pull-out. Provided the volume traction is
sufficient to allow this multiple cracking process to occur, the strength of the composite
depends on the fibres alone and Equation 2.5 app hes.
Fig. 2.2.
Tensile stress
strain curves for fibre reinforced brittle
matrices
predicted
by the ACK
theory (full
line),
and file bending response calculated from
iiem
(broken lines) (Laws, 1983)
In bending, while the beam behaves in a linear elastic manner the neutral axis is in the
centre of the beam and the nominal stress given by simple beam theory is equal to the
6
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 44/220
actual stress in the beam. W hen the tensile stress in the surface of the beam exceeds the
elastic limit it can no longer inc rease hnearly with increasing strain but will follow the
tensile stress / strain response of the material; and the nominal stress calculated from
simple beam theory is no longer equal to the actual stress in the beam (Fig. 2.3).
ompression
Netural oxis
TefTsion
troin distribution
tress distribution
Fig.
2.3.
Theoretical
model
applicable
to
low modulus
fibre-reinforced
cement composite atflexuralfailure
(Swift, 1979)
The bending curves calculated from the tensile curves in Fig. 2.2, are also shown in Fig.
2.2 (broken tines). At the critical volume fibre fraction (v ,,;,) the fibres support the stress
after the matrix has cracked in tension, and there is a long multiple cracking region (curve
A). Over this region the bending m oment continues to rise (curve A') and the ratio of
nominal bending strength or mod ulus of rupture (MOR) to ultimate tensile strength
(UTS) is high. As the volume fibre fraction increase s, the composite becom es more stiff
and the MOR / UTS ratio decreases. At high volume fractions the tensile curve
7
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 45/220
approaches that of the fibres alone and the MO R / U TS ratio approaches unity. Thus a
relationship between bending strength and fibre volum e fraction having the appearance of
a mixture rule can arise although the tensile strength / volum e fibre fraction relationship
depends on the fibre contribution alone (L aws, 1983).
Swift and Smith (1979) suggest that direct tensile strength cannot be significantly
improved by low modulus fibres within the hmits of strain acceptable in a tensile member,
whereas for flexural strength it is theoretically possible to obtain a large increase resulting
from the inclusion of low mod ulus fibres in the composite. They have theoretically
explained and demonstrated em pirically the improvemen t of flexural strength for sisal
slivers reinforced cement.
2 1 3 Basics of fracture mechanics
When the tensile strength of a brittle material is reached in a structure, cracking will occur.
The study of the conditions around and in front of a crack tip is called fracture mechanics.
Fracture mechan ics was first studied for brittie materials such as glass. The first
applications to concrete appear to have been made by Nev ille (1959).
The application of fracture mechanics to concrete structures has provided new ways of
understanding and modeling phenomena which could be treated empirically before.
Fracture mechanics refers to the analysis of the fracture of materials by the rapid growth of
pre-existing flaws or cracks. Such rapid (or even catastrophic) crack growth may occur
when a system requires sufficient stored energy that, during crack extension, the system
releases mo re energy than it absorbs. Frac ture of this type (often referred to as fast
28
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 46/220
fracture) can be predicted in terms of energy criterion (Rom ualdi, 1963; Bazant, 1985;
Bentur, 1990).
If we consider an elastic system containing a crack and subjected to external loads, the
total energy in the system, U, is
U
=
(-Wi + U E) +
Us
(2.6)
Where,
-Wi, UE
and
Us
are the work due to the applied loads, strain energy stored in the
system and surface energy absorbed for the creation of new crack surfaces, respectively.
A crack will propagate when dU/dc < 0, where dc is the increase in the crack length.
Using this theory, one can derive the Griffith equation, which gives the theoretical fracture
strength for brittle, linearly elastic materials,
(jj,r = {2Eys/ncy (2.7)
Where, Ci^u c and js are the stress at first crack strain, one half of crack length and the
surface energy of the mater ial. This is the basic equation of linear elastic fracture
mechanics (LEFM).
If we define a parameter
Gc
-
2 Ys
= critical strain energy release rate, then the equation
(2.7) becomes,
(jancf = (EGcf
(2.8)
That is, fracture will occur w hen, in a stressed ma terial, the crack reaches a critical size (or
when in a material containing a crack of som e given size , the stress reaches a critical
value).
29
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 47/220
Altematively, we may define a parameter Kc =
(JIJTTC) ^
= critical stress intensity factor.
K.^
= EGc
(2.9)
Kc
has the units of N/m- ^, and is often referred to as the fracture toughness (not to be
confused with the term toug hne ss , which is used to refer to the area under the load-
deflection or stress-strain curve).
The LEFM parameters,
Gc
and K.c, are one-parameter descriptions of the stress and
displacement fields in the vicinity of a crack tip. In much of the early work on the
apphcations of fracture mechanics to cement and concrete, it was assumed that they
provided an adequate failure criterion. How ever, later research showed tha t even for those
relatively brittle materials. L EFM could only b e apphed to extremely large sections (eg.,
mass concrete structures, such as large gams). For more ordinary cross-sectional
dimensions, non-linear fracture m echanics parameters provide a much better description of
the fracture process.
Fibres enhance the strength and, more particularly, the toughness of brittle matrices by
providing a crack arrest mechan ism (see also section 2.1.6). Therefore, fracture mechanics
concepts have also been applied to model fibre reinforced cem ent composites. Mindess
(19 ) has reviewed the difficulties in modelling cem ent composites based on the fracture
mechanics approach. LE FM might be adequate to predict the effects of the fibres on first
cracking. How ever, to accoun t for the post-cracking behaviour (which is responsible for
the enhanced toughness of fibre-cement com posites), it is essential to resort to elastic-
plastic or non-linear fracture mechan ics. A measu re of toughness (ie., the energy absorbed
during fracture) can be obtained from the area under the stress-strain curve m tension. The
fracture mechanics concepts which could provide a more precise measure of toughness of
3
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 48/220
fibre reinforced cement composites include the crack mouth opening displacement
(CMOD), R-curve analysis, the fictitious crack model (PCM), and various other
treatments, all of which model (either implicitly or explicitly) a zone of discontinuous
cracking, or process zone, ahead of the advancing c rack. These approaches provide
fracture param eters which are, at least, dependent on the fibre content, whereas the LEFM
parameters (G.:: or Kr) are most often insensitive to fibre content. It might be added here
that, while the J integral has often been used to describe these systems, theoretically it
cannot be applied to com posite systems such as fibre reinforced concrete, where there is
substantial stress relaxation in microcracked region in the vicinity of the crack tip.
In the investigation of the fibre-crack interactions using fracture m echanics concepts, the
crack suppression, stabilisation and fibre-matrix de-bond ing, three distinct issues must be
considered.
2 1 4 Fibre critical fracture length
The fibre critical fracture length is defined as twice the length of fibre embedment which
will cause fibre failure during pull-out. Th e fibre critical fracture length ,
l^.
can be
calculated from equation (2.9), assuming fibre strength (Cpj), fibre diameter ( 0 and the
shear stress (T.) developed at the interface are
all
uniform.
l,= <7fyd/2r, (2.9)
Andonian et al.
(1979)
calculated the critical fracture length of P.radiata fibres in WFRC
composites to be between 18 and 23 mm, and as the measured length of the fibre is about
3.5 mm, fibre fracture was not possible. Dav ies (1981) and Coutts (1982b) had observed
31
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 49/220
fibre fracture during WFRC com posite failure and concluded that the apparent fibre
critical fracture length must be
less
than 3.5 mm.
The conflicting reports on the predominance of fracture or pull-out of wood fibres from a
cement matrix prompted by Morrissey and Coutts (1985) to study a model system
consistmg of sisal slivers emb edded in cement and protruding from one end of the cement
matrix. Abou t 200 such samp les were tested under tension. It was found that the slivers
did not behave in the manner pred icted for uniform cylindrical fibres. After a break of the
elastic bond between the fibre and the m atrix, the fibre started to
puU
out. The force
resisting pull-out was not proportional to the length of embedment but was dominated by
the highest local resistances present due to the fibre morphology. As pull-out p roceeded,
anchor spots developed and the force required ro se or fell in an apparently random
manner. When the anchorage was too strong to be dislodged by the max imum force the
fibre could carry, tensile fracture of the fibre occurred (Fig. 2.4).
There was a critical fracture length of embedm ent, which for the sisal slivers was
approximately 30 mm . W hen the embedment was shorter, fibre tended to be pulled out,
(Fig. 2.4) and when it was longer they tended to break. This critical fracture length is
not the length for which a uniformly distributed frictional stress reaches its critical values
under the maximum sustainable tensile
load,
as is commonly assumed (Equation 2.9), but
it corresponds to the length around which the probability of a local stiong anchorage
becomes high.
32
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 50/220
4>
C
s
5
10
5
,
f r cture
pullout
10 15
20
• xt nsion
mm)
Fig. 2.4. Tensile load-extension curves for different failure modes of sisal silvers embedded in cement
(Morrissey, 1985).
The critical fracture length of 30 mm for sisal slivers corresponds to an aspect ratio of
110
± 50, which is comparable w ith the aspect ratio of
P .
radiata fibres. This would
support the observation tha t
P .
radiata wood pulp fibre cement composites frequently
experience fibre fracture during failure.
2.1.5 Fibre aspect ratio
Equation 2.4 and num erous studies of steel fibre and glass fibre reinforced cements and
concretes have indicated that the major factors affecting flexural strength are the fibre
volume and aspect ratio of the fibre
(l/d).
Higher values of either
lead
to higher values of
flexural strength (Hannant, 1978).
33
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 51/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 52/220
fibres are broken (Page, 1985), and so
cr,;,
is a
Hmiting
factor and not / /d. When tested
wet, the longer N Z flax fibres produ ce strong er samples than the short P. radiata fibres.
The hydrogen bonds between fibres or betw een fibre and matrix are destroyed (by
insertion of water molecules between the b ridging
hydroxyl
groups) (see 2.2.3), so more
flax fibres (long fibre) can be loaded up to failure. In keeping with this, we find the very
short E. regnans fibre reinforced ma terials are weak w hen tested w et or at RH test
conditions (50 ± 5% relative humidity, 22 ±
2°C)
as short fibre is pulled out and cannot be
loaded to failure.
2 1.6 Fracture toughness
As discussed in section 2.1.3, that the post-cracking ductility imparted to the composite by
fibre addition can be considerable. Gordon (1976) states: The worst sin in an engineering
material is not lack of strength or lack of stiffness, desirable as these properties are, but
lack of toughness, that is to say, lack of resistance to the propagation of cracks . The
origin of fracture toughness in WF RC composites was claimed to come m ainly from fibre
pull-out (85% - 90%) (Andonian, 1979), although later studies by these same workers
(Mai, 1983) considered fibre fracture must be of importance in agreemen t w ith other
researchers in the field (Davies, 1981; Coutts, 1982b; Fordos, 1986).
A generalised theory has been proposed by M artson et al. (1974) where the specific
fracture resistance {R) is given by the sum of; the toughness due to fibre pu ll-out (Rp.o),
redistribution of stresses
(RrS)
and fracture of surfaces
(i?,).
If the fibres are pulled out
rather than fractured, then it seems appropriate to neglect stress redistribution Ry. as a
component contributing to the specific work of fracture (R). Thus
R =
R,,„+R,
(2.10)
35
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 53/220
Where,
Rpo.
=
0.41vffx/12d
(2.11)
Rs
= VfR.
+
Vr Rr„
+ (0.41
VjflRif
/d
(2.12)
Rr,„ R f and Rif are the fracture energies of the cem ent mortar m attix, fibre and fibre-matrix
interface respectively. Normally, R f « R,,, and Rif = R ,„ (Martson, 1974) so that equation
(2.12) is simplified to
Rs = 0.41vflRn /d +
v„,R,,, (2.13)
The specific work of fracture R is thus given by
R = 0.41vffr/ 12d + [v,, + 0.41vfl / d]Rr (2.14)
Toughness measurem ents can be conducted in several ways (H ibbert, 1982); impact testers
such as Charpy or
Izod
which invo lve stored energy in a pendulum , calculating the area
under stress - strain curves, and the use of fracture mechanics involving stress intensity or
similar parame ters, or more practical tests such as dropped balls or weights, etc. Some of
these techniques are m ore suited to particular com posites, but all have limitations w hich
render elusive the well-defined material properties useful to engineers and material
scientists.
Mindess and Ben tur
(1982)
studied the fracture of W FRC products and found that
saturated samples were weaker and more com phan t than air-dry specimens. It was found
that the wet samples were completely notch-insensitive, while tire air-dry specimens may
be slightiy notch-sensitive. Notch sensitivity is a requirement for the application of hnear
elastic fracture mechanics to cement composites, and so these workers concluded that
LEFM could not be used for WFRC materials.
36
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 54/220
Mai and Hakeem (1984a, b) have studied the slow crack growth of WFRC composites for
both dry and wet conditions, using a doub le-cantilever beam system. The results were
analysed using
K-soIutions,
and compliance measurements within the framework of
LEFM. It was concluded that LEFM concepts can be used for WFR C products.
The use of LEFM requ ires a linear elastic hom ogeneous m atrix. The introduction of fibres
into matrix, to achieve ductility, mus t by their very na ture result in a heterogeneous
material with a non-linear stress-strain curve after m atrix cracking.
The fracture toughness results given in Tab le 2.1 for a range of NFR C composites have
all been obtained by the m ethod of measuring the area bounded by the load / deflection
curve. As the materials are non-linear and non-elastic, this approach is useful in that the
total recorded energy will include the con tributions of the w ork of fracture of the m atrix,
the debonding and frictional slipping of the reinforcement, and any strain energy released
by fibre fracture.
For fibre cements and concretes the matrix wo rk of fracture is virtually that of the
unreinforced cement or concrete and is less than 50 J/m^. Therefore it is assum ed that any
improvement m composite toughness
will
depend on whether the fibres bridging the crack
are able to support the load previously cartied by the mattix, and on whether the fibres
break or pull out of the matrix.
Fibre pull-out is the most common mode of failure for steel and glass fibre cements and
concretes. This is because , to incorporate sufficient fibre into the formulations without
7
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 55/220
fibre tangling, the fibre aspect ratio cannot be high enough to exceed the critical fracture
fibre length.
It has been recorded that wood fibres can be loaded into the ma trix material with fibre
volumes in excess of
v „.>j
and can be fractured during failure, hence the apparent critical
fracture length is exceeded . This behav iour does result in relatively high levels of
fracture toughness (Table 2.1) but does not lend itself to a conventional form of theoretical
analysis. The explanation of such properties is discussed in terms of fracture mechan ics
and bonding in section 2.2.
Recentiy, Hughes and Hannant (1985) have studied the reinforcement of Griffith flaws in
WFRC in an attempt to explain their behaviour, and concluded that stresses developed are
significantly higher than would be expected using the mixture rule theory.
It is hoped that as commercial production of NFRC becomes a global activity, more
interest will be shown in the theoretical explanation of how this system works and hence
in developing ways to op timise its use in design.
2.2 Bonding and m icro stru ctu re of NF RC composites
As well as the physical properties of the fibre and the matrix, a major factor w hich
controls the performance of a composite is the type and arrangem ent of bonds linking the
two materials.
8
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 56/220
According to com posite theory, the interface (that is the region of intimate contact
between fibre and matrix) plays the dual role of tiansmitting the stress between the two
phases and of increasing the fracture energy of the composite by deflecting cracks and
delocalising
stress at the crack tip .
The interfacial bond itself can be physical or chemical in natu re, or a combination of both.
Too strong a bond between fibre and matrix results in a brittle material which has strength
whereas a weak bond results in a tough material lacking strength. The mechanical
performance of a NFR C composite is therefore directly related to the nature and properties
of the
fibre-matrix
interface.
2 2 1 Chem ical bonding
The chemistry and morphology of the matrix material has been well documented (Lea,
1976) and will not be considered further, apart from stating that cement is strongly
alkaline (pH > 12) and presents metal hydroxy groups at its surface, such as -Ca-O H, -Si-
OH, -Al-OH and -Fe-OH (due to hydration and hydrolysis of silicates, aluminates and to a
lesser extent ferrites of calcium tha t are present in the cem ent ma trix). Cellulose fibres
such as wood fibres contain covalent hydroxyl groups, -C-OH, either phenolic (from
residual lignin) or alcohohc (from the cellulose component) and carboxyhc groups, 0=Q
OH, due to oxidation of end groups. Hydrogen bo nding and / or hydroxide bridges may
play a major role in the bonding of NFRC com posites. The chem ical implications wiU not
be considered quantitatively; it suffices to say that hydrogen bonds may form between
fibres or between fibres and matrix.
39
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 57/220
Coutts and co-workers (Coutts, 1979a, b) considered the possibility of using chemical
pretreatments of the wood fibres to enhance the bond between fibre and cemen t. The most
acceptable theory for the develop men t of coupling agents is the chemical bonding
theory , which suggests that the coupling agent acts as a link between fibre and matrix by
the formation of a chain of covalent chemical bonds (Fig. 2.5).
Although small improvements in composite mechanical performance have been recorded
from the use of pretreated fibres, the costs of such operations are currently considered
prohibitive. The initial hypothesis that some form of coupling agent or additive was
needed to achieve bonding stemmed from the general belief that the bond between wood
fibre and cement would be weak. It has subsequen tly been suggested that this may not be
the case (Coutts, 1984b) and that the presence of hydroxyl groups on the surface of both
wood and cement may be sufficient for adequate chemical interaction to take place via
hydrogen bonding.
2 2 2 Mechanical bonding
As well as the chemical bonding aspects of a fibre, the physical bonding potential must
also be considered. Much of the theoretical data on fibre reinforcemen t is based on
smooth cylindrical fibres of uniform sharp and dimensions (Section 2.1).
4
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 58/220
0 MX
(n-l)
{n-2
Fig,
2.5. Possible
coupling mechanism between wood
fibre
and cement matrix (Coutts, 1979a).
Maximum fracture energy is often achieved if frictional energy is dissipated via fibre pull-
out. Wood fibres are relatively long compared to their diameter and hence have aspect
ratios of say 60 - 200 (depending on whether they are hardwood or softwood, early wood
or late wood fibres), but, more im portantly, the fibres are hollow and can be collapsed to
ribbons and at the same time develop a helical twist along their length (like a cork-screw).
When fibres such as these are used to reinforce a brittle matrix, an asym metrical process
will be taking place during pull-out (after interfacial debonding has occurred). In classical
41
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 59/220
pull-out of straight fibres (glass, steel, etc.) the forces are symmetrically distributed around
the fibres; in the case of the contorted w ood fibres, the leading edge of the hehca l fibre can
experience considerable compressive stresses resulting in a ploughing action which can
damage fibre or matrix resulting in increased fracture surfaces and hence increased
fracture energy. Similar observations were recorded by Bentur t al (1985a,b) when they
studied the physical processes taking place during the pull-out of steel fibres, of various
geometry, from Portland cement.
Coutts and co-workers (Coutts, 1982a, 1984a) reported that mechanical refining or beating
of wood fibres resulted in improved flexural strength from autoclaved mortars reinforced
with such fibres. This phenomenon can again be discussed in terms of mechanical
bonding in that the externa l surface of the fibre is unwound and the fibrils so formed
offer extra anchoring points by which the fibres can accept stresses from the matrix and so
become more effective reinforcing elements. Although refining the fibre can assist in
mechanical bonding, its main value is in improving the drainage rate and solids retention
in the commercial Hatschek process (Anon, 1981; Coutts, 1982a).
MicheU
and Freischmidt (1990) studied curly fibre in the cem ent and silica matrix at the
aim of improving fibre and matrix bonding . The use of curly fibres in reinforced cement
and silica sheets gave sheets with improved wet interlaminate bond strengths, relative to
sheets prepared from conventionally treated fibres but had littie effect on the values of
modulus of rupture and fracture toughnes s.
Mechanical bonding has been discussed in other systems, such as asbestos fibre cement, in
which Akers and Garrett (1983a) showed that asbestos can be fiberized like wood fibres,
42
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 60/220
steel fibres can be kinked, as we have no ted in the w ork of Bentur t
al
(1985a), or
polypropylene can be fibrillated into a nethke form, as reported by Hannant and co
workers (1978).
2 2 3 The effect of moisture on bonding
The effect of moisture on the stieng th properties of NFR C com posites is of importance.
Earlier in WFRC research, variation in test conditions produced variations in test results,
and it was evident that standard cond itions must be adopted. Coutts and co-workers
reported that as the moisture content of a WFRC test specimen was increased, the flexural
strength of a given sam ple decreased while the fracture toughness increased (Coutts,
1982a). These observations have been confirmed by Mai
t
al (1983). This behaviour is
common to both air-cured (Coutts, 1985) and autoclaved (Coutts, 1982a, 1984b) samples
containing a range of
cellulosic
fibres, be they softwood (Andonian, 1979; Cou tts, 1983a),
hardwood (Coutts, 1987a), abaca (Coutts, 1987b) or NZ flax (Coutts, 1983c).
Coutts and Kightiy (Coutts, 1982b, 1984b) suggested that these observations could be
supported by the hypothesis that hydrogen bon ds and / or hydroxide bridges play a major
role in the bonding and hence in the mechanical performance of WFRC composites.
Wet or dry, a wood fibre has about the sam e tensile strength, bu t its stiffness is
considerably lower when wet. Thus a dried WFRC composite has stiff, highly contorted
fibres (see section 2.2.4) locked into a rigid cem ent ma trix which could be bonded at the
interface by a large number of hydrogen bonds or hydroxyl bridged sites. This system
when stressed can transfer the stress from the matrix to the fibres via the many interfacial
43
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 61/220
bonds, and hence sufficient stress may be passed on to the fibre, after the matrix has
cracked, to cause the reinforcing fibre to fracture under tensile load.
On the other hand, in a moist sample, the hydrogen bonds b etween fibres or between fibre
and matrix are destroyed (by insertion of water m olecules between the bridging hydroxyl
groups); and, at the same time, the cellulosic fibres are swollen by water absorption and
have become less stiff Unde r stress this system allows the fibres to move relative to each
other or to the matrix. How ever, due to the pressure of swelling and the highly contorted
assemblage of fibres, considerable frictional forces are developed . If the forces are
effective over sufficient length of a fibre, they can result in the fibres being loaded to
failure; however, the number of fibres that pull out without failure is higher than when the
sample is dry, and hence the observed values of fracture toughness for w et samples is
higher than for dry samples.
2 2 4 Microstructure of NFR C composites
The examination of wood-cement composites by scanning electron microscope (SEM) is
relatively recent. Ahn and
Moslemi
(1980) examined the manner in which wood particles
and Portland cement bonded together in cement pa rticle-boards and decided that
mechanical interlocking plays a significant
role.
This interlocking is due to crystal growth
during the hydration of cem ent. A similar crystal interlock ing effect has been reported for
wood fibre reinforced plaster products (Coutts, 1987c).
NFRC composites have been examined by different research groups with strong interests
in the micromechanical behaviour of such composites. In a num ber of these SEM studies
of NFRC materials, the microstructural features of the fracture surfaces of the broken
44
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 62/220
composites were described, with emphasis being placed on the surface of the fibre and
how it had failed (Davies, 1981; Coutts, 1982b; Mai, 1983; Pavithran, 1987; Akers, 1989).
More recently, interest has turned to the ma trix, and in particular, the interfacial region
adjacent to the fibre.
In a study of air-cured WFRC com posites, containing Kraft or CTM P
(chemithermeomechanical pulp) fibres, Davies
et
al (1981) proposed that the fracture
surfaces indicated that both fibre fracture and fibre pull-out were taking place during
loading to failure under am bient cond itions. And onian
et al.{l919 ,
studying autoclaved
WFRC sam ples, stated that fibre p ull-out was the m ain source of fracture toughness (80-
90%) even though the samples had been dried in an oven at 116°C for 24 h. Mindess and
Bentur (1982) also considered that a pull-out mechanism was the main process taking
place when commercially produced WFRC samples containing 20% by mass of wood
fibres were tested. These workers used only photographic evidence , stating that at 18 X
magnification (their highest), lack of focus restricted their observations due to the
unevenness of the surface. As the diameter of a wood fibre is approximately 1 5 - 4 0 frm,
the observation of end fractures would be difficult by optical means.
SEM studies by Coutts and Kightiy (1982b, 1984b), using autoclaved WFRC samples,
complement the earlier findings w ith air-cured s amp les, namely, that failure takes place by
mechanisms of fibre fracture and fibre pull-out. M ore importantiy, this study showed that
the relative importance of these mechanisms is very dependent upon the moisture content
of the test sample (Cou tts, 1984a). Figure 2.6, 2.7 and 2.8 show the SEM s of fracture
surfaces of WFRC samples obtained from tiie same formulation when tested dry, wet and
at 50 ± 5% RH . M ost of the fibres protrud ing from the fracture surface of a sample
45
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 63/220
preconditioned in an oven at 105°C for 24 h before testuig have fractured ends (Fig. 2.6).
By contrast. Figure 2.7 shows the fracture surface of a sample which has been
preconditioned in water for 48 h before testing and ind icates considerable fibre pull-out,
although fibre fracture is still very obvious (Fig. 2.7). Figure 2.8 shows the fracture
surface of the sample conditioned at near ambient conditions (50 ± 5% RH and 22 ± 2°C).
Both fibre fracture and fibre pull-out appear to have taken place. Higher magnification
(Fig. 2.8b) shows that considerable damage to some fibres of relatively short length has
taken place while other fibres have been pulled out of the matrix. M ai
et a/.(1983),
on re
examining their earlier work, are now of the opinion that fibre pu ll-out is not the major
source of toughness for WFR C, bu t indeed that fibre fracture is of considerable
importance.
The significance of mo isture content, as evidenced in the above SEM work, has been
adequately described in the section 2.2.3.
Fig. 2.6. SEM showing fracture surface of WFRC preconditioned at 100 - 105°C for 24h (Coutts, 1984b)
46
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 64/220
Fig. 2.7. SEM showing fracture surface of WF RC precondifioned by soaking in water for 48h (Coutts,
1984b)
Fig. 2.8a. SEM showing fracture surface of WFR C precon ditioned at 50 ± 5% relative humidity and 22 :
2°C (Coutts, 1984b)
7
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 65/220
Fig.
2.8b. As (a) but
higher magnification
Most of the foregoing studies have related to the condition of the fibre after failure and
rather little has been said of the most important aspect of the composite - the interfacial
region. In studies of air-cured cement matrices reinforced w ith aggregate particles
(Bames, 1978), steel (Pinchin, 1978; Bentur, 1985a,b) or glass fibres (Stucke, 1976;
Singh, 1981) detailed reference has been m ade to an interfacial region with structure and
porosity different to the bulk cement paste. This interfacial zone has, in several cases,
been considered to be 50 - 100 pm thick.
In the cases of aggregate, steel or glass fibre there is a general similarity of behaviour. At
the surface of the reinforcement a dense layer, consisting largely of hexagonal lamellar
crystals of portiandite
(calcium hydroxide or CH), forms and rephcates the topography of
the reinforcement over much of its surface a rea. At discontinu ity in this dense layer,
inclusions of hydration products such as calcium silicate hydrate (CSH) gel, ettringite and
large crystals of CH occur.
8
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 66/220
With increasing distance from the reinforcement surface, the proportion of CH decreases,
and a relatively porous layer (in comparison to the bulk of the matrix material) forms for
some distance before gradually becoming more dense and merging into the
bulk
matrix.
Fig. 2.9. SEM showing cem ent surface at interface contains dense m atrix with some discontinuities (Coutts,
1987d).
•^^
v,,; »
^» ^ y
Fig 2.10. SEM shows fractured fibres with dense m aterial from bulk of matrix up to fibre wall (Coutts,
1987d)
49
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 67/220
An SEM study of WFRC composites which focused on the region of the wood fibre-
matrix interface (Coutts, 1987d,e), revealed that the general characteristics associated with
the interfaces of other fibre reinforced cement composites w ere not observed. Coutts
noted that the interface between wood fibre and m atrix is usually dense and replicates the
surface of the fibre, although discontinuities do occur, especially at higher fibre loadings
when normal packing of the fibres becomes more difficult (Fig. 2.9). Again, if one looks
at the intimate area of contact betw een fibre and matrix (Fig. 2.10), no obv ious zone of
weak matrix material exists at the interface. It was stated that during fabrication of WFRC
composites the samples were placed under pressure to compact the structure and reduce
the water - cement ratio. It was hypothesised that, unlike the glass or steel fibres, w hich
are not compressible, the hollow w ood fibres are compressed, and after the pressure is
removed the water level immediately adjacent to the fibre is lowered even further as the
sponge-like fibre draws excess water back into
itself. This would reduce the occurrence of
voids at the interface of fibre and matrix particles, because less free water would be
present, and produce a homogenous dense ma trix material from the bulk of the material
right up to the point of contact with the fibre.
Another example of a dense interface is found m the work of Akers and G arrett (1983b)
who studied the microstructure and failure mode of die fibre-matrix mterfacial region in
asbestos fibre reinforced air-cured cemen t. Also reported was a mu tual interlocking of
asbestos fibres witii the cemen t hydrate, as had been observed with W FRC. But, more
importantiy, energy dispersive X-ray measurements showed no evidence of a calcium
hydroxide enriched zone adjacent to the interface, as had been reported for other fibre
cement ma terials. The study proposed that fiberizing asbestos fibres increased the fibre
surface available for water absorption, which helped to distribute the water-filled voids in
5
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 68/220
the interfacial region. Thus the nature of the fibre-cement interface need not be the same
as is described for materials such as glass, but appears to depend on the characteristics of
the reinforcing fibre.
In the case of TM P pulp reinforced composites (Coutts, 1986 and Chapter 9), the high
temperature of the autoclave (> 160°C) coupled w ith the alkali conditions of the matrix
material (pH > 12) results in the extraction of polysaccharides and wood acids from the
reinforcing fibre (see section 3.3), which, in the case of Kraft pulp have been removed
during pulp preparation. These extractives contribute to poisoning of the cement (Singh,
1979;
Thom as, 1983) and coating on the fibre surface thus causing poor interfacial bonds.
Hence, TMP pulp-cement com posites have low strength and poor fracture toughness (see
section 1.1.4).
51
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 69/220
Chapter Three:
The Nature of Plant Fibres
3 1 Fibre Classification
Natural plant fibre can be classified in to wood fibre and non-w ood plant fibre, which
consists of:
(a) Seed hairs;
(b) Bast fibres - fibres derived from the bark of dicotyledons, which include herbaceous
plants, shmbs, and trees;
(c) Leaf fibres - fibres derived from the vascular bundles of very long leaves of some
monocotyledons. Leaf fibres are also know n as hard fibres because they are more
lignified than bast fibres;
(d) Grass fibres - these are another group of monocotyledonous fibres where the entire
stem together with the leaves are pulped and used in paperm aking. Such pulps are
composed not only of fibres, but of other cellular elements as well.
Woods are grouped into two main classes, namely softwood and hardwood. The names
hardwood and softwood are based mainly on timbers produced in the northern hemisphere.
More correctly, the softwoods or coniferous types (pines, firs and spruces) are called
gymnosperms
and the hardwoods (gums, oaks and ashes) are all classed as
angiosperms.
The hardwood-softwood grouping has little mean ing on a world scale, as some hardw oods
(such as balsa wood) are extremely soft.
5
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 70/220
3 2 Structure of Natural Plant Fibres
Natural plant fibres can be derived from wood, bast, seed
leaf
and grass, needless to say
there are far too many plants to describe m this thesis. There are four types of tissue in
natural plants. They are parenchyma, fibre, tracheid and vessel (or
pore).
However some
plants do not contain all of these four tissues, such that softwood does not contain vessels
(or pores). Each type of tissue serves one or mo re special functions: the parenchym a cell
conduct and store food and water; the fibre's main role is to provide m echanical support;
tracheids and vessels (or pores) conduct w ater and dissolved m ineral salts from the roots
to the leaves as
well
as providing mechanical support.
In general terms it can be said that the fibre cells, which are themselves composite
structures, have a cylindrical or ribbon-like shape made up of different layers with a
hollow centre (lumen). The fibre can vary in leng th from less than 1 mm to greater than
250 mm. The diameter can be from less than 5 pm to greater than 80 pm (see section
3.5.1.1). When the fibres are separated from each o ther they can collapse flat, or the
lumen may remain open, if the walls of the fibres are thick. Fibres may develop a spiral
twist along their length, which can be of significance during fibre composite failure.
Plant fibres contain cellulose, a natural polymer, as the main material of reinforcement. In
a simplified description we can say chains of molecular cellulose are held together by
hydrogen bonds to form microfibrils, which in tum are held together by amorphous
hemicellulose and form fibrils. The fibrils are assembled in various layers of differing
thickness at different angles of orientation to build up the intemal structure of the fibre, the
main reinforcing e lement of interes t to our research (see Table 3.5). Figure 3.1 and 3.2
show schematic structure of wood fibres and bamboo fibre, respectively.
53
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 71/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 72/220
lack of performan ce of certain fibres can be attributed to brea kdo wn of these aggregates,
due to the alkalinity of the matrix materials, and not to the fibre cell itself
Fig. 3.3. Scanning electron micrograph of
a
cube of eastern white pine microtomed on three surfaces. Note
the arrangement of longitudinal
fi-acheids
(tr) in radial files and the smicture of the
rays.
In this species resin
canals
(re) are
rather prominent features (C6t6, 1980).
55
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 73/220
The increase in size of a natural plant varies with the seasons and climatic conditions.
Such as in trees, during late autumn and w inter, there is littie or no growth, but growth is
at a maximum in spring. The spring or early w ood usually has wood cells with larger
diameter and thinner walls than those cells formed during periods of slow growth -
summer wood or late wood (Fig. 3.3).
As will be seen later in this thesis, fibres with thick cell walls have different papermaking
characteristics com pared w ith thinner - walled fibres. Consequently, the ratio of later
wood fibres to early wood fibres can hav e a big influence on the abihty of a given wood
fibre source to act as a reinforcing element.
3.3 Chemical Composition and Durabili ty
The chemical com ponents which constitute m ost of the mass of natural plants are
cellulose, hemicellulose and lignin (Table 3.1). Cellulose is made up of thousands of
glucose molecular units joined in long chain s, and represents 40 - 45 % of plant. It
contains 44.4 % carbon, 6.2 % hydrogen and 49.4 % oxygen, and is relatively unaffected
by alkalis and dilute acids.
Lignin occurs in the form of very complex chain polymer of phenolic building blocks
consisting of about 65 carbon, 6 hydrogen and 29 % oxygen, and is virtually
impossible to dissolve without first breaking it down into simpler substances. Lignin
comprises between 22 and 30 % of plant and varies in chemical composition and amount
in different species, being somewhat more abu ndan t in softwoods than in hardwoods.
6
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 74/220
Table 3.1 Chem ical com positions of natural plant fibres (Atchison, 1983)
Type of fibre
Stalk-straw-rice
-wheat
-barley
-oat
-rye
-cane-sugar
-bamboo
-grasses-esparto
-sabai
-reeds-phragmites communis
Bast -seed flax tow
-seed flax
-kenaf
-jute
(1)
Leaf -abaca (Manila)
-sisal (agave)
Seed hull -cotton time rs
Woods -coniferous
-deciduous
Cross & Bevan
cellulose
43-49
49-54
47-48
44-53
50-54
49-62
57-66
50-54
54.5
57.0
75.9-79.2
47
47-57
57-58
78
55-73
53-62
54-61
Alpha
cellulose
28-36
29-35
31-34
31-37
33-35
32-44
26-43
33-38
44.75
45.1-68.5
34
31-39
60.8
43-56
80-85
40-45
38-49
Lignin
12-16
16-21
14-15
16-19
16-19
19-24
21-31
17-19
22.0
22.8
10.1-14.5
23
14.5-18.7
21-26
8.8
7.6-9.2
26-34
23-30
Pentosans
23-28
26-32
24-29
27-38
27-30
27-32
15-26
27-32
23.9
20.0
6.0-17.4
25
22-22.7
18-21
17.3
21.3-24
7-14
19-26
Ash
15-20
4.5-9
5-7
6-8
2-5
1.5-5
1.7-4.8
6-8
6.0
2.9
2.3-4.7
5
1.7-5.0
0.5-1.8
1.1
0.6-1.1
<1
<1
Silica*
9-14
3-7
3-6
4-6.5
0.5-4
0.7-3.5
0.69
2.0
* The content of tramp (as distinguished from inherent) inorganics may be greatiy increased by mechanical
harvesting.
Hemicelluloses act as a m atrix for the cellulose microfibrils, are of relatively low
molecular weight and are soluble in alkalis. They occur as about 15 to 30 % of the weight
of the natural plant.
All the chemical com ponents of natural plants are formed from the sugars produced in the
leaves by photosynthesis. The water obtained from the roots is combined with carbon
dioxide from the atmosphere, by complex biochemical processes dependent on chlorophyll
which is able to absorb from sunHght the energy needed, to form simple sugars. When
these sugars are conveyed from the leaves to the plant cells, they are changed into the
much more complex cellulose, hemicellulose and lignin.
57
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 75/220
In addition to cellulose, hemicellulose and hgnin , natural plan t also contains a wide range
(5 - 30 %) of com pounds known as extractives which can be extracted by solvents. These
materials, contribute to colour, density, durab ility, flammability and mo isture absorbency,
and include polyp heno ls, oils, fats, gum s, wa xes, resins and starches.
These extractives plus hemicellulose and lignin can inh ibit or retardate the hydration and
strength development of Portiand cem ent. In the case of TM P
pulp
reinforced composites,
the high temperature of autoclave (> 160
°C)
coupled with the alkali conditions of the
matrix material (pH > 12) results in fibre chemical degradation and extraction of the
fibres. These extractive s con tribute to poison ing of the cem ent and coating on the fibre
surface. Hen ce, TM P pulp-cemen t composites have poor mechanical performance (see
section 1.1.4 and 2.2.4).
Natural fibres are susceptible, under certain conditions, to biological deg radation, the
action of such decay resulting in, amongst other things , strength loss. How ever, in the
fibre cement system, chemical degradation w ould be m ore significant than biological
degradation due to high
alkah
environm ent (pH > 12). Sisal fibre (aggregate form) has
acceptable initial stiength. W hile, immersed in
lime solution, the fibre suffers a 74%
reduction in stiength due to significant chem ical degradation (Nilsson, 1975). Thus
composites reinforced with aggregate fibres have poor durability due to fibre degradation
(Lola, 1986).
58
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 76/220
3 4 Preparation of Natural Plant Fibres
3 4 1 Pulping
Natural fibre can be prepared by m eans of pulping. As stated, natural plant consists of
parenchyma, tracheid, vessel (or pore) and fibre tissues. The fibre cell contains cellulose,
hemicellulose and lignin. The purp ose of pulping is to separate the fibres from the plant
so that they are suitable for paperm aking and com posite reinforcement.
Wood pulp fibre is the plant fibre of mo st com mercial importance at the present time.
There are a number of ways of producing pulp which will be summ arized briefly in the
following sections.
3.4.1.1
Chemical pulping
Chemical pulps are mad e by heating the wood chips at high temperatures (about 170 °C
with a solution of chemicals that dissolve most of the lignin w hich bonds the fibres
together in the wood. The chemicals dissolve carbohydrates as well as lignin removing
about half of the wood subs tance , yield is typica lly 45 - 55 per cent. It is not possible to
remove all the lignin during chem ical pulping , therefore all unbleached chemical pulps
contain some residual lignin and are brown in colour.
The pulp is washed with water to remove pulping chemicals and dissolved wood
components. M ost modern pulp mills have a chem ical recovery system in which these
washings are concentrated and then burnt in a specially designed furnace to recover the
chemicals for re-use in the pulping process and to generate heat energy.
59
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 77/220
The Kraft process is used to produce a major po rtion of the world's chemical pulp.
Sodium sulphide and sodium hydroxide are the active ingredients which attack the lignin.
The process operates under alkaline conditions, and penetration of the wood is more rapid.
It is applicable to all species and so has a wide mdustrial appeal, bu t it creates considerable
effluent disposal problems and releases an unpleasant odour into the surrounding district.
The soda-anthraquinone (soda-AQ) process uses sodium hydrox ide with a small quantity
of anthraquinone as pulping chem icals. Pulps by this process are brown in colour and are
no easier to bleach than Kraft pulps. In mos t respects soda-A Q pulps are similar to Kraft
pulps.
The process offers no advantage with regard to bleachability of the pulps, but the
absence of sulfur offers the possibility of using new chemical recovery systems eg, the
Direct Alkah Recovery System (D ARS). The successful developmen t of DA RS may
make it possible to produce soda-AQ pulp in economically viable m ills of a smaller scale
than is currently necessary
If
the Kraft p rocess is used.
The alkaline conditions cause less harm to the cellulose than the acidic conditions of the
sulphite process, since ceUulose is more easily hydrolysed by acids than by alkalis.
The use of chemical pulps in W FRC products removes the problem of extractives
which, interfere w ith the cure of autoclaved products and fibre - matrix bonding resulting
in materials of low perfonn ance (Coutts, 1982b).
3.4.1.2 Mechanical pulping
Papermaking pulps can be manufactured by mechanically separating wood into its
constituent fibres. Chips are first steamed to soften the Hgnin and to make the separation
60
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 78/220
of the fibres easier. The chips are fed into a refiner w here they are ground between two
plates. Altematively billets may be fed onto a grinding stone.
After this treatment the freed fibres are screened and pum ped to storage. The amount of
pulp retained from mechanical processes is usually higher than 95 per cent as only the
water soluble material in the wood is removed.
Mechanical pulping in its various forms has been claim ed as the pulping method of the
future (Kurdin, 1983). This method can solve the problems of limited w ood supply, of
restricted capital, of environmental restrictions and of efficient utilization of mixed forests.
In its expansion from groundwood pulping, m echan ical pulping has advanced into the
domain of chemical pulping blurring what was on ce a clear line of distinction - there is
now refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemimechanical
pulp and chemithermomechanical pulp (CTMP).
Mechanical pulping is not without its weaknesses. M eans mu st be found to reduce the
energy consumption, increase the strength of the pulp and improve light stability. The last
factor is important in paper m anufacture, bu t not in the use of the pulp as a reinforcement
in cement systems etc
The principle of chemical and mechanical p ulping is scheduled briefly in Figure 3.4.
61
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 79/220
Fig.
3.4.
The principle of chemical and mechanical pulping
3.4.1.3 Non-wood pulping
Some non-wood resources provide excellent papermak ing fibres. Some bast fibres such as
jute, flax and kenaf can be produced by a retting m ethod, which allows bundles of fibres to
be freed from cellular tissue surrounding them by the combined action of bacteria and
moisture, then the fibrous material is crushed, washed and dried. Decortication, a process
used for leaf plants such as sisal and abaca, involves crushing and scraping the
leaf to
remove cellular tissue, then washing and d rying.
Besides these mechan ical process , almost all natural plant fibres can be prepared by the
Kraft pulping process. In countiies where wood is scarce, it is quite comm on to find
cereal straw, bagasse (from sugar cane), bamboo and similar materials being used as a
source of papermakmg fibre. These fibres have roughly the same quality as hardwood
fibres, sometimes a bit less because the fibres have been damaged during harvesting /
62
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 80/220
processing, but usually adequate as a com ponen t of printing and writing papers (Li, 1992).
However, there are a few difficulties that inhibit non-wood fibres from being used more
often as a source of papermaking fibres (McKenzie, 1992).
The first difficulty is that one can harvest the crop at only one time of the year. This
means that a who le year's supply of raw material must be collected and stored within a few
weeks. Furthermore, it canno t merely be cut and stored, but must be protected from decay
immediately after harve sting . Also, the prospec t of a crop failure (for whatever reason)
and the subsequent closure of a mu ltimilhon dollar manufacturing operation for lack of
raw material is enough to deter the average investor.
Many agricultural residues h ave a high silica content (rice straw and bamboo are notorious
in this respect) which dissolves in alkaline pulping liquors and redeposits throughout the
pulping plant. The bu lkiness of straws and similar materials compared to wood chips can
also be a problem, as the produc tivity of a digester depends on the amount of raw m aterial
which can fit into it. This m eans that the cost of a mill designed to produce straw pulp
will probably be much higher than the cost of a wood chip mill capable of producing the
same amount of pulp of the sam e type .
Overall, the cost of producing pulp from non-wood material is high. It can only be
justified economically if the
pulp
is of superior quality, such as is the case w ith pulp
produced from textile or cord age grade fibres. It is significant that in many countries
where wood is scarce but agricultural residues are plentiful, it is comm on to import wood
pulp rather than to produce non-woo d pulp from m aterials such as wheat straw. In other
countries, straw pulp and the like is only used because of
govemment
restrictions on the
63
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 81/220
importation of wood pu lp. It is estimated tha t China and India produce nearly half of the
annual world total of 10 million tonnes of non-wood based paper.
3 4 2 Refining or be ating
Refining or beating can be defined as the me chanica l treatment of pulp carried out in the
presence of water, usually by passing the suspension of pulp fibres through a relatively
narrow gap between a revolving rotor and a stationary stator, both of which carry bars or
knives aligned more or less across the line of flow of the stock. The term beating is
usually apphed to a batch treatment of pulp suspension, whereas refining is used when
the stock is passed continuously through one or m ore refiners in series.
It should be po inted out that the refining of chemical pulp does not produce the same
effects as it does on mechanical pulp . Chem ical pulps are relatively pure cellulose, with
the hydroxyl groups accessible. In mechanical pulps the hydroxy l groups are blocked by
the presence of
lignin.
The refining of mechanical pu lp is really a completion of the
process of disintegration of fibre bundles down to individual fibres.
Changes observed in fibre structure as a result of the m echanical action on the fibrous
material can depend on the type of refiner, the refining conditions used, the fibre type
(hardwood or softwood) and the pulp (mechanical or chemical). However the main effects
which are observed can be classified into four areas:
(a) Intemal fibrillation or delamination
(b) External fibrillation of the fibre surface
(c) Fines formation
(d) Fibre shortening
6
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 82/220
Intemal fibrillation effects (a) are difficult to observe under a microscope, but they can be
considered by analogy with a piece of rope. Rope is a helical wrap of strands which
themselves are hehcal wraps of fibres. If one twists a rope in the direction of the helical
wrap the rope becomes stiffer; likew ise, if the twist is in the opposite direction the rope
unwinds (or delaminates) to open up the stmcture and becomes floppy ; such is the case
with intem al fibrillation. Th e main effect of intemal fibrillation is to increase fibre
flexibility and swelling. The fibres may also undergo excessive curling and twisting.
External fibrillation (b) is easily observed by scanning electron microscopy (Fig. 3.5). The
fibrils or fibrillar lamellae attached to the fibre surface can vary widely in size and sharp
(but the process is again like unrave lling a piece of rope at its surface).
The last stage (c) of extemal fibrillation is the peeling off of the fibrils from the fibre
surface with the formation of fines. Depending on the forces acting on the fibre during
refining, more or less of the fibrils will be removed from the surface of the fibre.
Fibre shortening (d) is the other
prunary
effect attributed to refining. An indication that
fibre shortening has occurred is the change observed in particle size distribution, which is
a result of the cutting action of the blades or discs in the machinery on the single fibres.
Refining plays an important role in producing a large surface area for fibre - to - fibre or
fibre - to - matrix (in the case of com posites) bonding and, more importantiy, can assist in
controlling the drainage rates of processing liquids during the fabrication of products.
This is one of natural fibre's main advantages compared to synthetic fibres such as glass,
steel, etc., in asbestos replacement.
65
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 83/220
Fig. 3.5. (a, top) Unbeaten fibre of P.radiata, com pared to (b) externally fibrillated fibre (Coutts, 1982b).
3 5 Properties of Natural Fibres
The significance of the properties of the natural fibres in the cement composite will be
studied through out this thesis. The physical and mechanical properties of natural fibres,
such as
fibre
size, morphology, surface charge, strength,
etc.,
play important roles in the
manufacturing process of fibre cements.
66
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 84/220
3 5 1 Physical properties
3.5.1.1. Physical dimensions
The physical dimensions of fibres are exceedingly unportant in the apphcation
of fibres as
reinforcement, be it in paper, med ium density fibre board or in fibre reinforced cement.
Hardwood fibres are much shorter (av. 1.0 m m) and narrower (av. 20 pm than softwood
fibres (av. length 3.5 mm). With softwoods there is a difference in fibre diameters,
between early and late wood (av. diameter 45
|Lim
and
13 |im
resp.). Hardwood fibres
generally have a higher relative cell wall thickness than do early wood softwood fibres.
This implies hardwood fibres are stiffer and have a greater resistance to collapse (see
Table 3.2).
Table 3.2 Typical softwood and hardwo od fibres phy sical dimensions (Coutts, 1988).
Fibre property
P. radiata E. regnans
Fibre length (mm)
Fibre width
(|im)
Early wood
Late wood
Cell wall thickness
(|am)
Early wood
Late wood
Type of cells (%)
Fibres
Vessels
Parenchyma
It can be seen from Table 3.3 and 3.4 that natural fibres have great variation in length,
diameter, lumen size and fibre wall thickn ess. Som e of the fibre properties for reinforcing
are better than those of wood fibres, for example bast fibres and
leaf
fibres have much
greater length than that of softwood fibres.
2 - 6, av: 3.5
45
13
3
5
> 9 0
/
< 1 0
0 . 5 -
10
2.5,
-23
av: 1.0
av:20
/
/
4
/
/
65
18
17
67
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 85/220
Chemical pulping processes reduce natural plant to pulp, which is then composed of
individual cells. For wood and m ost of non-wood pulp, the small percentage of non-fibre
cells can be washing away and the pulp contains mainly pure fibre cells. How ever, the
pulps of grasses (cereal straws, sugar cane bagasse, bamboo,
etc.
contain fibres as well as
a great variety of other cells, such as parenchyma , epiderm al and vessel segm ents. These
segments and pitted cell act as a filler rather than as a reinforcing material in the cement
based composites.
Table 3.3 Some non-wood fibres physical dimensions (Strells, 1967)
Fib re s
Seed cotton
Hairs coir
fiax
hemp
Bast jute
ramie
kenaf
abaca
Leaf sisal
NZflax
Lengtii (mm)
10-40 (18)
0.3-1.0(0.7)
9-70 (33)
5-55 (25)
1.5-5(2)
60-250 (120)
2-6 (5)
2-12 (6)
0.8-8 (3.3)
2-15
(7)
Width (|im)
12-38 (20)
12-24 (20)
5-38(19)
10-51(25)
10-25 (20)
11-80(50)
14-33 (21)
16-32 (24)
8-41 (20)
5-27(15)
Shape
ribbon-like
cylindrical
"
II
II
"
II
"
"
Lumen size
broad
"
fine
broad
fine
Wall thickness
thick
thin
thick
Table 3.4 Some grasses pulps physical dimensions (Strells, 1967)
cereal straw
sugar cane
bamboo
F i b r e
L. (mm)
0.68-3.12 (1.48)
0.8-2.9**
0.8-2.8(1.7)
1.45-4.35(2.7)
2.8-3.26^'
W
|jm)
* 6.8-23.8(13.3)*
27-34**
10.2-34.1 (20)
6.8-27.3
(14)
20.5-40t
P a r e n c h y m a
L. (mm) W.
(|am)
0.45 130
0.85 140
0.25 65
V e s s e l
L. (mm) W. (|im)
1.0 60
1.35 150
/ 100
* thick wall
** thin wall
pitted fibres
3.5.1.2
Surface potential and surface area.
The Hatschek process is the major manufacturing method for fibre cement products and,
as have been shown (section 1.1.2), is very much akin to a papermaking machine. The
retention of cem ent particles by asbestos (or its replacement) and drainage properties of
68
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 86/220
the sheets during the dewatering stage are param oun t unp ortance, hence fibre surface
potential and surface area are of considerable significance.
Zeta-potential - the electrokinetic charge on a colloidal particle - has been discussed in
many areas of science. When the particle is a wood fibre it has been postulated that the
zeta-potential plays a major role in the process of papermak ing,. The zeta-potential is a
controlling parameter in filler and fines retention, dramage and pulp flow behaviour (Arno,
1974).
Work
in the field of paper science (Britt, 1974; Herring ton, 1986) shows quite
dramatically that zeta-potential is not a measure of surface charge, and cannot be used for
comparison of the surface charge of even very similar ma terials, let alone two ma terials as
distinct as asbestos and wood fibre. In this discussion it suffices to say that in the case of
cellulose fibres, the present evidence indicates that d issociation of ionic groups is the main
source of the charge. At very high pH va lues, dissociation of the hydroxyl groups can
contribute to the particle charge, but at low pH values the charge must be due to
dissociation of carboxyl groups or sulphonic acid groups, depending on the pulp yield and
on whether a sulphite or sulphate pulp is used. A cellulose fibre dispersion is negatively
charged at all pH values. W hen cationic hydroxylated m etal species, such as occur in
cement particles, are present, charge reversal may occur, as has been established in the
case of wood fibres in the presence of papermaker's alum (A mo, 1974).
3 5 2 Mechanical prope rties
The use of natural fibres in paper, paperboard, and fibre reinforced composite m aterials
has created a need for better characterization of fibre m echanical prope rties.
69
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 87/220
Unfortunately, researchers often report properties of the fibre when in fact they are
studying aggregates of fibres with very different properties to those of the individual fibre.
For example, bast and leaf fibres have been widely used in the textile and cordage
industries. Ten sile strength in the textile indus try is defined in terms of tenacity which is
the breaking load per unit mass per unit length; for co rdage applications, it can be defined
as the breaking length w hich is the length of fibre which can theoretically support its own
weight when suspended at one end.
Most of the pioneering work in natural fibre (wood) testing was carried out by researchers
from the paper industry, and much of the data on fibre properties interpreted from tests
carried out on paper sheets. The information on single fibres is limited, although new
techniques in recent times are providing more data as time passes (McKenzie, 1978).
3.5.2.1 Modulus of elasticity and tensile strength
Theoretical calculations suggest the modulus of elasticity of cellulose could be as high as
150 GPa. Experimental values for single fibres vary with change in fibril angle, drying
restrains and defects but range betw een 10 - 100 GPa (Fig.3.6 and 3.7).
Like elastic mo dulus, the tensile strength of single fibres is dependent on the helical angle
of the S2 layer and the presence of defects. Page and co-workers (Page 1970, 1971; Kim,
1975) have reported a value of 2000M Pa for defect-free black spmce fibres with a zero
fibril angle; this figure should be considered as a max imum value. A more realistic value
would be in the range 500 - 1000 MPa for selected fibre fractions carefully prepared to
minimize fibre damage. Jayne (1959) found values for tensile strength between 350 -
1000 MPa.
70
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 88/220
Fig. 3.6. A pulp fibre consists of cellulose fibrils in largely parallel array, embedded in a matrix of lignin and
hemicellulose. The majority of the cellulose fibrils form a steep spiral aroun d the axis of the fibre, and it is
this structure which gives to the fibre its mechanical strength
Typtal average
fibril angles for
comm erci l pulps
D )
C
a
h .
^
0)
w
c
0)
0)
i l
10
iber strength\fibril angle data iskenfrom
_ _ „ .
Page et
al
Pulp Paper Mag Can.,73, 8):T198 lynrJ
20 30 40 50
Fig. 3.7. The maximum strength of a pulp fibre is limited by the inherent prop erties of the cellulose fibril
modified by the spiral angle of the fibril about the fib re axis. The actual s trength exhibited by the fibre in
relation to this limiting strengtii is then determined by the presence or absence of defects and dislocations
which produce weak spots where premature failure is induced.
71
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 89/220
Considerable data from zero-span tensile testing of paper sheets have been equated to
tensile values for single fibres. It would appear from all the available evidence that the
likely average tensile strength of fibres com mercially p repared from reasonable quality
wood by normal methods would be not
less
than 700 MPa. Strengths of 800 - 900 MPa
might be anticipated if the wood supply is carefully selected to contain a large proportion
of late wood. This is not likely to be readily achieved in Australia or New Zealand w here
the main softwood species is young
P radiata
(McKenize, 1978).
The more com mon fibres, such as abaca, sisal, flax, kenaf and bamboo, range in tensile
strength from say 50 - 500 MPa, with densities of about
1.2-1.5
g/cm'. The elastic
modulus of plant fibres can range between 5 - 100 GPa (Coutts, 1992b). Unfortunately,
fibres are often characterized as an aggrega te of fibres rathe r than as an individual fibre,
thus information is limited in the scientific lite rature. Table 3.5 lists some fibre stmcture
and strength values, it should be interpreted with c are.
Table 3.5 Structure and strength parameters of non-wood fibres (Mukherjee, 1986)
Fibre Cellulose% Spiral angle cen L.( mm ) L / W UTS (MPa) Elongation%
Seed
Bast
Leaf
coir
fiax
hemp
jute
remie
banana
sisal
43
71
78
61
83
65
67
45
10
6.2
8.0
7.5
12
20
0.75
20
23
2.3
154
3.3
2.2
35
1687
906
110
3500
150
100
140
780
690
550
870
540
580
15
2.4
1.6
1.5
1.2
3.0
4.3
Pulp fibres strength properties can be reflected by com paring the pulp for tear, burst,
tensile and other sheet properties at different levels of laboratory beating. The properties
can be plotted and compared at different freeness levels; at different levels of beating time;
72
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 90/220
and against each other. When properties are plotted against each other, the freeness, or
beating time, or sheet
bulk,
is indicated. These curves can then be compared with curves
similarly obtained from other pulps, in order to assess the effects of type of fibrous raw
material, and of cooking, bleaching and refining procedure, upon final strength.
c lao
4 eo 8
too
TAPPI IMETRICI
BURST FACTOR
Curve A: Abaca bast fiber) urtbleached sulfate
pulp
B:
U S A
Southern pine unbleached sulfate
pulp
C:
U S A
Douglas-fir unbleached sulfate
pulp
D:
U S A
southern market hardwood
bleached sulfate pulp
E: Bamboo unbleached sulfate pulp
F:
U S A
West Coast bleached sulfite pulp
G:
Esparto unbleached soda pulp
H: Reed unbleached sulfate pulp
I: Estimated average mixed tropical hard
woods unbleached sulfate pulp
J:
Kenat whole stalk unbleached soda
pulp
K: Eucalyptus hardwoo d) unbleached sul
fate pulp
L:
U S A aspen hardwood) bleached sul
fate pulp
M: Rice straw unbleached soda pulp
N: Wheat straw unbleached soda pulp
O: Bagasse unbleached soda pulp
P: Bagasse bleached soda pulp
Q: Kenaf woody core material unbleached
soda pulp
R: Kenaf bast ribbon unbleached soda
pu p
S:
Sisal pulp
Fig. 3.8 Laboratory beating - strength tests on chem ical pulps from various wood and non-wood plant fibres.
Digits superimposed on the curves represent Canadian Standard Freeness tests 100 (Atchison 1983).
Figure 3.8 depicts one group of such curves, it can be seen that there are tremendous
differences in strength between the pulps prepared from these different raw materials. For
73
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 91/220
example, the tear strength of abaca (curve A) is outstan ding; and the tear strength of most
of the softwood pulps is high com pared to that of hardwood p ulps and most of the non-
wood plant pulps. Mixed tropical hardwood pulps com pared favourable w ith plantation
hardwoods and with temperate zone hardwoods.
Also
to be no ted is the strength of kenaf
bast fibre (curve R); its properties are comparable to those of softwood fibres. By contrast,
the strength of kenaf core pulp (curve Q) is very low . Its initial freeness is also very low,
meaning that it is a very slow-draining pulp.
3.5.2.2 Fibre flexibility
The flexibility of a fibre is of great importance during the preparation stage of a composite
material and also during the process of com pos ite failure. Certainly w et fibre flexibility
has been known to be important for paper man ufacture, but the property was seldom
measured quantitatively.
Tam Doo (1982) and Kerekes (1985) have developed a single-fibre flexibility test method
which ehminates the disadvantages of earlier methods. This has enabled the researchers to
measure the effect of wood species and pulping conditions on fibre flexibility and confirm
quantitatively what has long been know n in a qualitative way . They showed that cedar
fibres are flexible (1.33 x 10 Nm ^), Douglas fir fibres are stiff (6.8 x 10 ^
Nm"-)
and
Southern pine pulp fibres are very stiff
(11.7
x
10"2
Nm '^). It was also shown that
mechanical pulps are 20 - 30 times stiffer than chemica l pulps from the same species. By
chemically treating mechanical pu lps, a marked decrea se in stiffness can be noted.
74
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 92/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 93/220
content, the torsional stiffness of fibres decreased by about 50 % (between 25% and 90%
RH at room temperature) (Nevell, 1984). This softening is related to the softening of the
hemicelluloses in the cell wall. In the longitudm al direction this effect (up to 90% RH)
results in a drop in modulus of only 11%, although the hemicellulose modulus is reduced
by a factor of a thousand . This is clearly a reflection of the fact that the stiff cellulosic
microfibrils are preferentially aligned along the fibre.
In many cases, a maxim um in fibre tensile strength and tensile modulus is found at low
moisture contents. W hen relatively dry, fibres show low tensile properties due to a poorer
stress distribution within their stra cture. Such an effect is no t restricted to natural fibres
but also occurs in synthetic fibres.
When single fibres are imm ersed in water, the relative long itudinal stiffness drops
considerably i.e. by about 70 -80% from the value at 50% RH . This has been explained as
being due to softening of the disordered zones of the cellulose microfibrils and also to a
reduction in cell wall cohesiven ess leading to a slippage between fibrils. The relative
decrease in modulus of
pulp
fibres, when immersed in water, is almost independent of
yield.
76
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 94/220
Chapter Four:
Air-cured and Autoclaved Bamboo Fibre Reinforced Cement
Composites BFRC)
Recently, there has been a trend to use natural cellulose fibres to replace asbestos fibre in
the fibre reinforced cement indus try. In Australia wood fibre P.radiata) has replaced
asbestos fibre as a reinforcement in comm ercial cem ent produc t since
1981.
This fibre has
a reasonably high m arket price. Thus considerable research effect has gone into the study
of fibre composites from fast growing cheap agricultural crops and crop residues,
especially for those countries with limited forest resources.
Bamboo is a rapid grown agricultural crop, which has good fibre qualities and is w idely
used in the paper industry throughout the Asian region. Although bamboo has been used
in various forms in the constmction industry, there is limited information in the scientific
literature conceming the use of bamboo pulp fibre. Sinha et al (1975) and Pakotiprapha et
al
(1983)
reported that air-cured bam boo fibre reinforced cement com posites had flexural
strength values close to 20 MPa, at a fibre loading of 10% by ma ss. How ever, there was
no report of the values of fracture toughn ess, which is as important a mechanical property
as strength or stiffness when considering building materials.
The Hatschek process followed by curing in a high pressure steam au toclave has been
commercially applied to the production of wood fibre reinforced cement (WFRC)
products. Steam curing at temperatures close to 180°C enables the replacement of
between 40% to 60% of ordinary Portiand cemen t by less expensive silica, the latter reacts
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 95/220
with the cement to form a calcium silicate ma trix of acceptable strength. The reaction is
completed w ithin 6 to 8 hours instead of 3 to 4 weeks requ ired w ith air-cured products.
This chapter discusses the preparation and mech anical and physical properties of air-cured
and autoclaved B FRC composite to establish their suitably as an alternative to wood pulp
fibre for asbestos replacement in fibre cement building products.
4.1 Expe rimen tal work
4.1.1 Materials
The bamboo fibre was unbleached Kraft pulp of Kappa No.26, and was prepared from
commercial packaging paper (Chang Jing Paper M ill, China). The bamboo species was
Sinocalamus ffinis Rendle) McClue.
The matrix was prepared from ordinary Portiand
cement or from equal proportions of ordinary Portland cement and finely ground silica
(Steetly brand, 200 m esh, washed quartz).
4.1.2 Fibre modification
The fibres used in the composites were obtained by soaking the commercial packing paper
overnight followed by disintegration for 10 min at a speed of 2850 RPM . After
disintegration, the fibres were subjected to 3 different modifications:
1.
vacuum-dewatering and crumbing [unbeaten bamboo pulp (400 Canadian Standard
Freeness {CSF})];
2.
beating in a Valley Beater to 100 CSF, then vacuum-dewatering and crumbhng [beaten
bamboo
pulp
(100 CSF)];
78
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 96/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 97/220
4.2 Air-cured bamboo fibre reinforced cement
4.2.1 Mechanical properties
Air-cured BFRC composites were prepared from two different bamboo pulps, one beaten
(100 CSF) and the other unbeaten (400 CSF). The physical and mechanical properties of
the BFRC composites are reported in Table 4.1. These properties are also compared with
those of both softwood and hardwood pulp reinforced cements in Figures 4.1- 4.4.
36-
32-
28-
24-
a
X
h
D
X 12-
W
-J
U
8
•
I
r
6 8 10
PERC ENT FIBR E (BY MASS)
X
^
•
•
P. radiata
E. regnans
unbeaten baniboo
beaten bamboo
<
12 14
Fig. 4.1. Effect of fibre content on flexural stiength for air-cured WFR C and BFR C
Figure 4.1, shows the variation m
flexural
strength, of BFRC composites, as the fibre
content is increased. This same graph contains published data for air-cured WFRC
reinforced with softwood
P.
radiata (Coutts, 1985) and hardwood fibres E.
regnans
(Coutts, 1987a). Flexural strength values for BFRC increased from about 10 MPa up to 22
MPa as the fibre content increased from 2 % - 14 % by
mass.
The flexural strength value
80
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 98/220
for unreinforced cemen t board was suggested about 9 MPa from the graphic. Unlike
softwood and hardwood fibres which indicated maximum values of flexural strength for
the composites at about 8 % by mass, the bamboo reinforced material was still mcreasing
in flexural strength at loading of 14% by mass for both the unbeaten and beaten pulps.
Table 4.1. Properties of air-cured bamboo-fibre-reinforced cement
Fibre (w%)
Unbeaten Pulp
2
4
6
8
10
12
14
Beaten Pulp
2
4
6
8
10
12
14
MOR (MPa)
10.1
±2.0
13.4±1.9
15.0±2.3
17.0±1.7
19.7±1.0
21.4±3.8
21.2±2.4
10.9±1.4
12.1±1.3
16.2±1.0
17.4±0.8
18.6±1.1
19.2±1.4
21.8±1.7
Frac. Tough(kJ/m2)
0.10±0.04
0.15±0.02
0.23±0.03
0.34±0.05
0.49±0.05
0.80±0.23
0.97±0.23
0.07±0.01
0.15±0.02
0.23±0.02
0.32±0.03
0.45±0.07
0.54±0.05
0J0±0.06
Void Vol
(%)
26.3±0.9
26.3±0.6
26.5±0.6
26.6±1.1
26.0±1.1
25.6±0.5
25.3±0.9
26.8±0.7
28.3±0.6
27.6±0.4
26.6±0.9
26.5±0.9
27.1±07
28.0±0.6
Water Abs.(%)
14.6±0.7
15.2+0.5
15.9±0.5
16.7+0.3
16.8±0.7
17.6±0.4
17.9±0.8
14.6±0.6
16.7±0.4
16.7+0.4
16.5±0.6
16.9±0.6
18.2±0.3
19.4±0.4
Density (g/cm')
1.81±0.03
1.73±0.02
1.67+0.02
1.59±0.06
1.55±0.02
1.46±0.03
1.42±0.02
1.83±0.03
1.69±0.03
1.65+0.03
1.61±0.03
1.57±0.05
1.49±0.03
1.44±0.04
*A11 composite were fabricated using ordinary portiand cement, air-cured for 28 days, tested at 50+5 per cent RH and 23±2°C.
* 3 standard deviation, sam ple size n=9 .
The fracture toughness values of BFRC increased with increasing fibre content (Table 4.1
and Fig. 4.2). At a fibre loading of 14% by mass the fracture toughness values w ere ~
1.0
kJm"2
and ~ 0.7 kJm 2, for samples containing unbeaten and beaten fibres respectively.
These values are low com pared to softwood and hardwood fibre reinforced W FRC 's at
fibre loading of 12% by mass which are 2.25 klm'^and 1.68 kJm' respectively (Fig. 4.2).
The initial indication is that bam boo fibres are unsuitable on their own as a reinforcement
for cement products as they lack the essential property of fracture toughness. The use of
hybrid fibre cement formulations containing both bamboo and
long
softwood fibres have
been investigated in an attempt to improve fracture toughness properties and will be
reported in Chapter 5.
81
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 99/220
2.50-
2 00
B
v>
W
1.50-
Z
I
o
o
W
1.00
0.50
0 00
X
P
radiata
*
£ .
regnans
• ur beaten baniboo
• beaten
b mboo
— 1—
12
8 10
PERCENT FIBRE BY MASS)
14
Fig. 4.2. Effect of fibre content on fracture tough ness for air-c ured W FR C and B FR C
An important observation with regard to the mechan ical performance of the BFRC
compared to softwood and hardwood W FRC 's is that the long softwood pulp fibres
produced superior properties of flexural strength (> 30 MP a) and fracture toughness (>
2.28 kJm"2) than either BFRC or hardwood WFRC, at similar loading of 8 % by mass of
fibre. This behaviour has been explained by the fact that the long softwood fibres (as
measured on a Kajanni F S-200) had a fibre length w eighted average of -2.8 mm and were
more able to contribute to the reinforcement and to the process of fibre pull-out during
fracture. The shorter bamboo fibres (average fibre length w eighted 1.1 m m) or hardwood
fibres (average fibre length weighted 1.0 mm) are not as effective as reinforcement.
However, this is not the complete explanation as BFRC and hardwood W FRC have
similar flexural stiength values (see Fig.
4.1),
yet BFRC materials have m uch lower
fracture toughness values than hardwood WFRC (see Fig. 4.2).
82
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 100/220
In natural fibre reinforced cement com posites fibre len gth plays a major role in the
mechanical performance of the material. At the same time other fibre properties such as
fibre coarseness, wall thickness (lumen size), fibre wall stmc ture and fibre strength
etc.
also effect the composite properties. For examp le in this study, bamboo fibre is much
weaker than softwood or hardwood fibre. Zero-span tensile index as measured on
handsheets made from pulps of bamboo,
P.radiata
and
E.regnans
were 71 Nm/g, 138
Nm/g and 138 Nm/g respectively. Bam boo fibre has a different m icrostructure from wood
fibre, there is a very small lumen (compa red to softwood fibres) and the fibre's primary
wall is easily pealed off during refining(Wai, 1985; Wang, 1993). Furthermore bamboo
pulp contains a considerable num ber of segmen ts and pitted cells (see section 3.5.1.1.),
which act as a filler rather than as a reinforcing material in the cement based composites.
The complexity of these parameters is currently being studied by investigating the
relationships between pulp fibre properties and the properties of the cement composites
derived from such pulps and some of the preliminary results
will be discussed later in this
thesis.
4.2.2 Physical properties
The physical properties of void volum e, water absorption and density are reported in Table
4.1. There appears to be littie difference in the physical properties of the composites
containing either beaten and unbea ten bam boo fibre. As the fibre content was increased
from 2 % - 12 % fibre by mass the water absorption only increases from 14 % to about
18
% by mass (see Fig. 4.3). It can be seen that as the fibre content increased the rate of
increase in water absorption was much lower for bamboo fibre containing materials than
for softwood (14 % - 25 %) and hardwood (18 % - 30 %) fibre reinforced products.
83
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 101/220
Likewise, the density of the bamboo reinforced materials decreased at a lower rate than
either softwood or hardwood WFRC (Fig. 4.4), while the void volume remained fairly
constant over the range of fibre contents studied.
32
8
24
z
o
ft.
o
i i
oa
<
20
16-
a
U 12-
<
X p radiata
«
E. regnans
•
unbeaten
bamboo
a
beaten bamboo
i 1 1 1
4 6 6 10
PER CE NT FIBRE (BY MASS)
— —
14
2
Fig.
4.3.
Effect of fibre content on water absorption for air-cured WF RC and BFR C
84
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 102/220
2 00
1.80-
1.60-
< 2
z
Q
1.40-
1.20-
X p
radiata
^ E. regnans
• unbeaten bamboo
o
beaten bamboo
10
— —
12
14
PERCENT nBRE(BYMASS)
Fig. 4.4. Effect of fibre content on density for air-cured W FR C and BFR C
4.3 Autoclaved ba m bo o fibre reinforced cem ent
4.3.1 Mechanical properties
4.3.1.1 Flexural strength
1.
Unbeaten fibre reinforced composites
Table 4.2 and Figure 4.5 show the variation in flexural sttength of BFRC composites, as
the fibre content is increased from 2% to 22% in steps of 2%. Figure 4.5 contains
reference data for autoclaved WFRC composites reinforced with Kraft P.radiata fibres, as
this fibre is comm ercially used in Aus traha and is the prefcrted fibre. Flexural strength
values for autoclaved BFRC composites increased from about 12 MPa up to 18 MPa as the
fibre content was increased from 2% to 14% at which point the stiength of the composite
products start to decrease due to poor fibre distribution throughout the matrix material.
85
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 103/220
This observation was
in
general agreement with the change
in
flexural strength observed
with autoclaved W FRC comp osites.
2. Beaten fibre reinforced composites
Beaten (100 CSF) and unbeaten (400 CSF) BFRC material gave similar flexural strength
values
of
approximately
18
M Pa
at
14% fibre (Table 4.2). With the WFRC composites
it
was found that beaten
radiata
(550 CSF) gave
a
maximum flexural strength
of
24.3
MPa
at
10% fibre loading. This was
an
improvem ent over products reinforced with
unbeaten fibres.
Table 4.2 Properties of autoclaved bamboo-fibre-reinforced cemen t
Fibre
(w%)
Unbeaten Pulp
2
4
6
8
10
12
14
16
18
20
22
Beaten Pulp
2
4
6
8
10
12
14
16
18
20
22
M O R ( M Pa)
13.3±0.5
13.2+1.8
13.8±0.9
15.5±1.0
16.0±0.8
16.5±1.2
18.3±1.6
17.2±1.5
17.7±1.0
16.1 ±0.6
14.9±1.3
12.1
±0.5
12,2±1.0
14.6±1.3
14.9+1.2
16.1 ±0.9
17.1±1.4
18.2±1.3
18.2±1.4
16.4±0.9
17.1±1.5
16.7±1.2
Frac . Tough(kJ/in2)
0.08±0.01
0.13±0.01
0.20±0.02
0.29±0.05
0.39±0.04
0.49±0.05
0.62±0.0I
0.77+0.07
0.96±0.14
1.08±0.11
1.20±0.31
0.10±0.01
0,13±0.02
0.22±0.03
0.29±0.04
0.40±0.04
0.50±0.02
0.50±0.01
0.56±0.06
0.82±0.08
0.97±0.08
1.03+0.11
V o i d
Vol (%)
35.7±0.5
38.0±0.9
39.1±1.0
43.1 ±2.2
45.1 ±1.7
45.7±0.9
45.4±4.9
48.6+1.4
49.2±1.2
50.8±0.8
50.9±0.6
34.2+0.6
37.7±].l
39.8±1.5
41.1+1.1
44.2±1.3
44.3+1.4
42.8±0.8
44.7±1.5
46.9±1.1
47.0±0.8
47.4+1.0
Water Abs.(%)
22.1
±0.5
24.9±0.9
26.7±1.0
30.7±1.1
33.9±1.5
35.9±1.3
36.6±6.5
41.1±2.3
42.8±2.3
45.1 ±2.1
49.0±2.3
21.0±0.6
24.8±1.0
27.6±1.4
29.1 ±1.1
33.1±1.6
34.1+1.6
32.5±1.2
35.9±2.0
40.6+1.8
40.9±1.1
42.2±1.4
Density
(g/cm.-^)
1.62±0.02
1.52±0.03
1.46+0.02
1.41±0.04
1.33±0.02
1.27
±0.02
1.26+0.11
1.18±0.03
1.15±0.03
1.13±0.05
1.00±0.05
1.63±0.02
1.52±0.02
1.44±0.02
1.41+0.02
1.34±0.03
1.30±0.03
1.32±0.03
1.25±0.03
1.16±0.04
1.15±0.01
1.12±0.02
*BFRC composites were fabricated using ordinary Portland cement and silica at the ratio of 1:1, autoclaved at
1.25MPa
steam pressure for 7.5h, tested at 50±5 per cent RH and 22+2°C. BFRC composites maximum flexural strength at 14%
by mass. 3 standard deviation, sample size n=9.
86
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 104/220
26
24
22
20
-3- 18
S 16
t 14
12
I
10
iS
8
6
4
2
0
^ : 2 4 J
unbeaten BFRC
beaten BFRC
screened BFRC
beaten WFRC
u
10 12 14
Percent Fibre (by mui
16
18
20
22 24
Fig. 4.5. Flexural strength as a function of percent
fibre
oading for autoclaved BFRC and WFRC composites
Table 4.3 Fibre weighted average length (mm)
beaten P. radiata unbeaten bamboo beaten bamboo screened bamboo
2 4 1 0
0.8
1 3
Bamboo has a fibre length less than half of that of
P.radiata
(Table 4.3). The bamboo
fibre used in this study was separated from com mercial packaging paper and there was a
high fines con tent in both the beaten and unbeaten pulps. The beaten and unbeaten
bamboo pulp fines (lengths < 0.4 mm) accounted for 32.7% and 28.8% of the mass
respectively (Table 4.4). This suggests httle damage occurred during beatmg.
Table 4.4 Fibre length mass distribution percentage
Lengtli (mm) longer than beaten P.radiata unbeaten bamboo beaten bamboo screened bamboo
0.05
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
100
95.6
90.0
81.8
72.1
61.3
48.9
36.8
28.0
99.7
71.2
51.6
32.2
19.2
10.9
5.9
3.1
1.7
99.7
6 3
42.1
22.6
11.4
5.3
2.5
1.3
0.6
100
94.8
74.3
51.1
30.3
15.4
7.6
3.6
1.6
Fibre length mass distribution percentage was analysed on
Kajanni
FS-200 fibre length analyzer. More than
15,000 fibres w ere measured and analysed. Length less than 0.4mm fibre mass distribution percentage for
p.radiata
imbeaten
bamboo, beaten bamboo and screened long bamboo pulp was 4.4%,
28.8%,
32.7% and
5.2% respectively.
87
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 105/220
3.
Screened fibre reinforced composites
Bamboo pulp was washed through
a
SomerviUe screen (0.83 mm hole size, yield 46%)
to
remove fines and to obtain the longer fibres
for
use
as
reinforcement. Com pared with
unscreened unbeaten fibre the screened fibre showed improved composite flexural strength
at the same fibre loading. The maxim um flexural strength of screened B FRC was 21.6
MPa
at
10% fibre loading (Table 4.5).
Fibre length can make
a
significant contribution to the composite flexural strength.
Softwood P.radiata fibre, and screened and unscreened unbeaten bamboo fibres, had fibre
lengths (length weighted average) was 2.4 mm, 1.3 mm and 1.0 mm respectively (Table
4.3).
The flexural strengths of materials reinforced with wood fibre, and screened and
unscreened unbeaten bamboo fibres were 24.3 MPa, 21.6 MPa and 16.0 MPa respectively
at 10% fibre
by
mass (Fig. 4.5).
Table 4.5 Properties of autoclaved screened long bamboo fibre reinforced cement
Fibre
(w%)
2
4
6
8
10
12
14
MOR (MPa)
13.3±1.4
14.8+1.5
]6.3±2.2
18.8±1.2
21.6+2.2
19.5+1.5
18.9±1.9
Frac. Tough(kJ/ni2)
0.10±0.01
0.23+0.02
0.37±0.06
0.54+0.12
0.71±0.13
0.79±0.06
1.09±0.14
Void Vol
(%)
39.1±0.8
41.2±1.0
42.6±1.0
42.6±0.9
42.7±1.4
46.6±0.6
47.0±0.9
Water Abs.(%)
24.7±0.8
27.7±1.1
3O.0±1.3
30.1+0.9
32.0±1.4
37.8±1.4
39.8±1.7
Density (g/cm')
1.59±0.02
1.49±0.02
1.42±0.03
1.42±0.07
1.33±0.02
1.23±0.03
1.18±0.03
*
Unbeaten bamboo pulp was screened on 0.83mm hole size Sommerville screen (yield 46%) fibre length weighted
average (Kajanni FS200 fibre analyser) was 1.3 mm and the freeness was 550CSF. BFRC maximum flexural strength
was at 10% by mass fibre loading, which was similar fibre loading to WFRC. 3 standard deviation, sample size n=6.
The flexural strength
of
unscreened BFRC materials (beaten and unbeaten) increased up
to
14% fibre before the max imum value was reached. This could
be
due
to
the fact that the
fine material (length < 0.4 mm ) offers littie reinforcement to the comp osite and so a
greater mass of pulp was needed to have sufficient numbers of long fibres. When fines
(lengths
<
0.4 mm ) w ere removed, maximum stiength w as achieved w ith fibre loading
between
8 -
10% , which
is
in keeping with the early study
of
WFRC composite.
The
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 106/220
percentage of screened bamboo pulp fibre lengths longer than 0.4 mm, was 94.8% of its
mass.
This is similar to the data for P.radiata pulp (Table 4.4).
A fibre length fractionation study of P.radiata wood pulp supports this belief that
fragments with length less than 0.3 mm, act more as a filler-diluent than as a reinforcing
fibre when used to make WFRC products (see Chapter 6). A similar behaviour regarding
the maximum flexural strength value with respect to fibre mass content (about 12%) was
noted for waste paper fibre reinforced cem ent products (Coutts, 1989). In that instance,
the high fibre content was required to prov ide sufficient mass of the longer reinforcing
fibres. This was due to the high fines content generated (lengths < 0.6 mm constituted
21.2% by mass) during processing and recycling.
There is httle difference between beaten and unbeaten BFRC composites with respect to
flexural strength, which contrasts with the observation reported in the earher research on
beaten and unbeaten P.radiata W FRC com posites (Coutts, 1984a). Similar behaviour to
that of autoclaved BFRC was found in the case of autoclaved NZ flax composite (Coutts,
1983c) and air-cured bamboo reinforced cement products (Coutts, 1994b). This might be
associated with the fact that the round and small diameter NZ flax and bamboo fibres do
not have the same ability to collapse as do softwood fibres which form flat ribbons.
Altematively, it may be due to the beating , generating fines w hich do not contribute to the
composites stiength.
89
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 107/220
4.3.1.2 Fracture Toughness
The mechanism that takes place when a fibre composite is loaded to failure include fibre
fracture and fibre pull-out. The latter can have considerable influence on the value of
fracture toughness. If the fibre is short then the energy required to pull the fibre through
the matrix, after the fibre to m atrix bond is b roken, is low and can contribute httie to the
dissipation of energy contained in the advancing crack. Therefore
the crack continues
through the sample and the material appears brittie.
2.6
2.4
2.2
2
cT 1 8
• unbeaten BFRC
—
beaten BFRC
— •
screened BFRC
beaten WFRC
52 45
10 12 14
Percent
Fibre (by mm)
16
24
Fig. 4.6. Fracture toughness as
a
function of percent
fibre
oading for autoclaved
BFRC and WFRC
composites
Figure 4.6 shows tiie increment in fracture toughness values of beaten and unbeaten BFRC
composites (from 0.10 kJ/m to 1.03 kJ/m and from 0.08 kJ/m to 1.20 kJ/m respectively)
for fibre loading from 2% to 22%. The same graph has reference data for WFRC
composite fracture toughness, which are seen to have higher values (Coutts, 1984a).
Screened BFRC composites which contain more long fibres are tougher [from 0.10 kJ/m^
to 1.09 kJ/m2 for fibre loadings from 2% to 14%, (Fig. 4.6)] than unscreened composites.
Unbeaten BFR C com posites are
sHghtiy
better than beaten ones at the high fibre loadings.
90
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 108/220
WFRC composites indicated better properties of fracture toughness than BFRC
composites in the three point bending load/deflection curve as shown in Figure 4.7. After
reaching the maximum load, the curve for WFRC composites went through a gradual
tailing off which indicated tha t a greater amount of fracture energy was needed to pull
the long fibres though the matiix. The curves for the BFR C com posites did not show such
behaviour and were more brittle. This is indicated by a sharp drop of f of load carrying
capacity in the load/deflection curve. Fracture toughness behaviour is related to fibre
length and fibre morphology, and comparison is only valid for specimens of the same
thickness.
Fig. 4.7. Typical
Load
/ Deflection graph for autoclaved
WFRC
and
BFRC
composites.
Beaten BFRC com posites did not vary greatly in fracture toughness values from those of
the unbeaten BFRC com posites. At high fibre loadings the beaten BFRC composites
showed slightiy lower values due probably to the increased amount of fines present in the
formulation (Table 4.4). As stated above, the presence of fines provide s less opportunity
91
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 109/220
for fibre pull-out being a major component in the mechanism of failure, and hence lower
fracture toughness values are observed (see Chapter 6).
4.3.2 Physical Properties
The changing proportions of the constituent fibres and matrix affect void volum e, density
and water absorption (Table 4.2 and 4.5). There is little difference in the density of the
composites when the bamboo fibre was beaten compared with the materials containing
unbeaten bamboo fibre (Fig 4.8). How ever, the same graph shows a slight decrease in
density, at a given fibre content, when the pu lp has been screened to remove the fines.
This effect is possibly due to the increase in fines, present in the unscreened pulps, which
allows closer packing of the fibres and m atrix, and hence less void volume in the
composite.
1.8
1.6
1.4
1.2
I
f
^
0.6
0.4
0.2
0
— n
• unbeaten BFRC
H J — beaten BFRC
—• screened BFRC
6 8
10 12 14
Percent
Fibre (by mui)
6
18 20 22 24
Fig. 4.8. Density as a function of percent fibre oading for autoclaved BFRC composites.
The relationship between water absorption and density is depicted in Figure 4.9. The
amount of water absorbed by the cellulose fibre reinforced cement composites depends on
92
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 110/220
their void volume and the amount of cellulose material present; both these parameters
have an effect upon density. Thus one would expect the density to decrease and the water
absorption to increase as the fibre content is increased, due to the nature of the
hydrophilic, low density bamboo fibres. At the same tim e the packing of fibres and matrix
becomes less efficient, as the fibre content is increased and so void volume increases
accompanied by decreased density and increased water absorption.
I s
0 9 1.1 1.2 1.3 1.4
Demity (g em- )
1.5
1.6 1.7
Fig. 4.9. The relationship between density and water absorption for autoclaved BFR C composite
4.4 Conclusions
Bamboo fibre s a satisfactory fibre for incorporation into the cement matrix. Air-cured
BFRC composites at 14% by mass had flexural stiength values of about 22 MPa.
Altematively, autoclaved products had strength values of about 18 MPa at same fibre
loading. However, the fracture toughness was low due to short fibre length and high fines
content of the bamboo pulp.
93
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 111/220
By screening out fines contained in the original bam boo pulp, autoclaved composites
flexural strength could be improved to greater than 20 MPa while fracture toughness
exceeded 1.0
kJ/m2
at a loading of 14% fibre .
In contrast to softwood fibre reinforced cement composites, beaten bamboo fibre
composites did not vary greatly in flexural strength and fracture toughness values from
those of the unbeaten fibre com posites.
94
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 112/220
Chapter Five:
Bamboo and Wood Hybrid Fibre Reinforced Cement
Composite Materials BW FRC)
As we have discussed in Chapter 4 com posites reinforced with bamboo fibre have lower
mechanical properties than composites reinforced with wood fibre due to relative short
fibre length of bamboo fibre. This study includes in blending fumishes of bamboo pulp
and long softwood fibre pulp to increase the av erage fibre length thus hopefully improving
composites mechanical properties.
Pulp blending idea is used widely in the pu lp and paper industry with the aim of
modifying the final product specifications. For exam ple, long fibre pulp blended
with short fibre pulp can improve short fibre paper p roperties of tear, tensile and
burst strength; short fibre
pulp
blended with long fibre pulp can improve long
fibre paper properties of formation and surface smoothness. Blended fum ish of
some pulps at certain proportion
woitid
even generate synergistic effect for some
paper properties (Kuang, 1992).
Over the years, the combined use of different types of fibres to optimise the performance
of a material has also been studied and com mercialised by a number of material scientists
and technologists. A considerable amo unt of work has been done in hybridising
polypropylene fibre, PVC fibres, glass fibre and cellulose fibre with the aim of improving
fibre-cement products fracture toughness and durabihty (Walton, 1975; M ai,
1980;
Simatupang, 1987; Gale, 1990). However, the poor temperature resistance of
95
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 113/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 114/220
content was measured. The wood pulp w as blended w ith bamboo pulp in a small mixer
into various proportions as shown in Table 5.1.
5.1.2 Fabrication and characterisation
Fibre cement composites reinforced with above prepared hybrid fibre were produced and
characterised as the method described in Appen dix A.3 and A.4. Air-cured samples were
reinforced with 6%, 8% and 10% of blended
pulp.
Whereas, the autoclaved samples were
reinforced with 8%, 10%, 12% and 14% of blended pulp.
2 5
i 2
•g 1 . 5
a
'o
5
0 5
20 40 60 80
Proportion of Pino in Total Pulp ( , by weight)
100
Fig. 5.1. Relationship between pine fibre proportion and furnish pu lp length w eighted average.
97
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 115/220
5.2 Results and discussion
5.2.1 Length and freeness of blended pulp
The value of length weighted average and freeness of blended pulps is listed in Table 5.1
and depicted in Figure 5.1 and 5.2. These fumishes were prepared from different
proportions of bamboo and pine pulp. Both fibre length and freeness of blended pulp
increased almost linearly as the proportion of pine fibre increased. For example, bamboo
pulp alone had length about 0.91 mm and freeness around 330 CSF; while, 60% bamboo
and 40% pine blended fumish had increased length to 1.54 mm and draining abihty to 470
CSF. As discussed m Chapter 5, BFR C had fairly poor strength and toughness values due
to the short length of bamboo fibres. Blended pulp has increased average fibre length thus
one would expect com posites reinforced with such hybrid fibre to have better mechanical
properties.
20
40 60 80
Proportion of Pine in Total Pulp ( , by w eight)
1
Fig. 5.2. Relationship between p ine fibre proportion and fiirnish pulp freeness value.
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 116/220
5.2 .2 Air-cur ed BW FR C compos ites
Results of air-cured BWFRC composites mechanical and physical properties are listed in
Table 5.2 and shown in Figure 5.3, 5.4 and 5.5, 5.6.
Table 5.1 Length and freeness of blended pulp
Proportion of Blended Pulp
Length Weighted Av. (mm) Freeness (CSF)
100B*+0P
90B+10P
80B+20P
60B+40P
40B+60P
20B+80P
OB+IOOP
0 91
1 22
1 31
1 54
1 82
2 05
2 37
330
390
410
470
540
630
680
* B and P represent bamb oo and pine pulps respectively. The numb er before B and P is the weight proportions of bamb oo and pine
pulp in the W ended fijrnish.
Table 5.2 Properties of air-cured BWFRC
Fibre (w%)
100B+ OP
6%
8%
10%
90B
+lOP
6%
8%
10%
SOB + 20P
6%
8%
10%
60B + 40P
6%
8%
10%
40B + 60P
6%
8%
10%
20B + SOP
6%
8%
10%
0B+ lOOP
6%
8%
10%
MOR (MPa)
17.812.3
18.1+1.1
18.5+2.0
18.8±1.7
20.4±1.8
20.111.5
17.911.5
21.0+2.2
23.312.0
20.311.5
21.3+1.9
22.711.2
22.9+1.4
24.411.9
25.811.8
23.811.8
26.612.7
25.012.4
24.311.4
25.511.5
26.510.9
Frac. Tough(kJ/m2)
0.8210.06
0.9710.05
1.24+0.22
1.1110.21
1.2610.17
1.34+0.09
0.9510.22
1.3510.19
1.6710.11
1.2110.20
1.3610.23
1.7810.25
1.4710.27
1.6910.38
2.38+0.58
1.4610.27
2.1510.33
2.4910.46
1.9710.21
2.3810.38
2.9610.37
Void Vol (%)
33.110.9
33.6+0.5
35.3+1.4
32.6+0.9
33.710.7
35.310.6
31.9+0.7
33.8+0.5
33.9+0.9
32.510.5
34.610.5
35.611.0
32.0+0.7
33.610.8
33.810.3
32.010.4
33.311.2
35.0+0.8
32.5+0.6
32.0+0.4
33.110.3
Water Abs.(%)
20.610.7
21.9+0.4
23.211.2
20.111.0
21.610.7
23.610.6
19.410.6
21.510.6
22.310.9
20.010.4
22.210.6
23.711.1
19.210.8
21.410.3
22.710.6
19.210.4
21.011.2
23.310.9
19.1+0.6
19.9+0.4
22.2+0.4
Density (g/cm')
1.6410.02
1.5610.01
1.5010.02
1.6310.04
1.5710.03
1.50+0.01
1.6410.02
1.57+0.02
1.5210.02
1.6410.02
1.5610.02
1.5010.03
1.6610.03
1.5710.02
1.5210.03
1.6610.02
1.5810.04
1.5010.03
1.6610.02
1.6110.02
1.5310.02
*A11
composite were fabricated using ordinary portiand cement, air-cured for 28 days, tested at 50+5 per cent RH and 23±2''C.
* 3 standard deviation, sample size n=9.
99
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 117/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 118/220
more pronounced than on the strength property. If tiie fibre is longer, more energy will
need to be consumed to pull the fibre out from the matrix, after fibre-mah-ix bond is
failure; thus, miproves composite fracture toug hness. Figure 5.4 shows that fracture
toughness value of composite increases very rapidly with increasing of long softwood fibre
proportion in the reinforcing fibres.
6 of total pulp
8 of total pulp
1 0 of total pulp
20 40 60 80
Proportion of
Pina
in To tal Pulp ( , by
waioht
1
Fig. 5.4. Influence of long fibre (pine) proportion on the air-cured comp osites fracture tough ness.
Influence of long fibre proportion on the strength and toughness properties of composite
can be understood in terms of fibre length. The relationship between fibre length and
composite flexural stiength or fracture toughness is shown in Figure 5.5 and 5.6
respectively. The observation w ill be seen more clearly in Chap ter 7. In that instance,
studies on the composites reinforced with various length modified pulps show the
significance of fibre length to the composites strength and toughness development.
101
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 119/220
0,5
1,5 2
Fibre Length Weighted A v. mm)
2.5
Fig. 5.5. Influence of fibre length on the air-cured com posites flexural strength at total 8% fibre content.
0.5
1.5 2
Fibre Length Weigh ted Av. mm)
2.5
Fig. 5.6. Influence of fibre length on the air-cured compo sites fracture toughness at total 8% fibre content.
The changing of wood fibre proportion of the constituent hybrid fibre is not expected to
affect composite physical property such as void volume, density and water absorption.
102
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 120/220
Table 5.2 shows fair constant value of composites physical properties if the total
reinforcing fibre content remains the same.
This work is very impo rtant for utilising natural fibres as reinforcing ma terial in fibre-
cement products. Most natural plant fibre (including some wood fibre) do not have good
reinforcing potential due to their short fibre leng th. As discussed above, short fibre
blended with certain proportions of long fibre such as Kraft pine would increase the
average fibre length thus significantly im prove its reinforcing ability. One of the
weaknesses of waste paper is its unsatisfactory length distribution w hen utilised as a
reinforcing material (Coutts, 1989). This weakness could be overcom e by adding a certain
amount of long fibre pu lp. On the other hand, one would modify the produc ts prope rties
by adding cheap short fibre to reduce the cost of fibre produ ction.
In WFRC manufacture p ractice, natural fibre (eg. softwood) requires some degree of
refining (beating) to enhance its reinforcing ability, more importantly to control the
drainage rates of processing liquids during the fabrication of products (Coutts, 1982a).
Unrefined (unbeaten) wood fibre has very high freeness value about 700 - 800 CSF.
Refining can reduce this freeness down to 500-550 C SF , which is preferred for the
manufacture proce ss. Such an effect could also be achieved by blending long fibre with
short fibre without refining (see Table 5.1 and Figure 5.2). It might be possible to reduce
the reinforcing cost by means of fibre blending. However, this idea needs further w ork to
confhm in both laboratory and pilot-plant scale before it is adopted by the industry.
5 3
Autoclaved BW FRC composites
As in the case of the air-cured BWFRC, tiie strength property of autoclaved BWFRC
composites also increased with an increase of long wood fibre proportion as seen in Table
5.3 and Figure 5.7, 5.8. However, strength improvemen t was not as high as that in the
case of air-cured BW FR C. Strength of air-cured com posites improved from 18.1 M Pa
reinforced w ith bambo o fibre alone to 26.6 M Pa reinforced with blended 80 proportion of
wood fibre at 8% of total fibre content, about 47 % increase. But in autoclaved
103
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 121/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 122/220
B of total pulp
1 0 of total pulp
1 2 of total pulp
1 4
of total pulp
20
40 60
roport on t Pina in Total Pulp
( .
by weight )
80
100
Fig.
5.7. Influence of long fibre (pine) proportion on the autoclaved composites flexural strength.
25
20
15
10
5 -
0.5
1.5
2
Length Weight Av. (mm)
2.5
Fig. 5.8. Influence of fibre ength on
the
autoclaved compositesflexuralstrength at total
8% fibre
content.
Fracture toughness value of autoclaved hybrid composites increased very rapidly with
increasing long fibre (wood) proportion as seen in Figure 5.9 and 5.10. This behaviour has
already been observed in the case of air-cured composites. Fracture toughness values of
105
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 123/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 124/220
Physical property of autoclaved products remamed
fairiy
constant at tiie same total
reinforcing fibre conten ts. But the density values was about 18% lower than that of the
air-cured counterpa rt. W ater absorption and void volume always have reversed
relationship
witii
density. Hence, higher density samples would have low water absorption
and void volume.
5.3 Theoretical predication and the experimental results
We have discussed early in section 2.1 that the composite strength and toughness can be
calculated from the equations (2.4) and (2.14) respectively.
Jt, = [a/p]
cr.,,;,v,„ +
0.82 aT)vf l/d)
(2.4)
R =
0.41vfft/
12d +
[vn,.
+
0.41vfl
/ d]Rr, (2.14)
Where
a,
p
T,
<7„,h
and
R„
are constants w hich can be determined experimentally. The
fibre diameter d is suggested to remain constant for certain types of fibre. Thus at a
constant fibre content
Vf
and v,„, the com posite strength increases linearly, wh ile the
toughness increases as a second o rder polynom ial with increasing fibre length. Refer to
equations (5.1) and (5.2).
Oj,
= Kn l) + Kn
(5.1)
R
=
K2i f}
+
K2:il) + K23
(5.2)
Where K-i, Kj-,
Kzu K22
and
K23
are constants.
The experimental results support the above theoretical prediction. A simple regression
analysis was performed on the data in Table 5.2 and 5.3. The results are presented in Table
5.4 which displays a relationship of fibre length, composite strength and toughness. The
relationship is valid as the statistical multiple coefficient
(R"")
is approximately 0.95.
107
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 125/220
Table 5.4 Regression analysis results
Fibre
(w%)
MOR (MPa) Fract. Toughness
(kJ/m')
Relation
R
Relafion
R/
Air-cured
6%
8%
10%
Autoclaved
8%
10%
12%
14%
5.29
L+12.3 5
5.74 L+13. 26
6.16
L+13. 48
4.12 L +9.98
4.47 L+10. 43
4.49
L+10 .68
5.44
L+-8.98
0.917
0.900
0.830
0.931
0.947
0.969
0.977
0.24 L -0.06 L +0.71
0.25
L^
+0.15 L +0.70
0.33
L
+ 0.24 L + 0.70
0.50L -0.81 L +0.64
0.35 L -0.19 L +0.30
0.89
L ^ -
1.56
L + 1.29
-0.45
L
+2.53 L-1.42
0.944
0.970
0.951
0.973
0.958
0.947
0.990
Table 5.1 shows that the blended pulp fibre length increased almost lineariy as the
proportion of pine fibre increased. Therefore the composite strengtii increased almost
hneariy with increasing proportions of long fibre content. On the other hand, the
composite fracture toughness improved as a second order polynomial function.
5 4 Conclusions
Bamboo pulp when blended with softwood pine pulp improves the fumish pulp's average
fibre length and drainage abihty. The improvement increased with increasmg the
proportion of wood pulp (long fibre).
The advantage of introducing the hybrid fibre was to improve and modify the composite
properties. In tiiis study increasmg the propo rtion of wood fibre caused improvement of
both air-cured and au toclaved composites mechanical prope rties, but had littie effect on
their physical properties. The improvement was m inimal but more obvious in the case of
the Stiength property of air-cured products than on the autoclaved counterpart. The
108
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 126/220
improvement in the property of fracture toughness was more pronounced than that of
strength for the same formulations containing hybrid fibres.
109
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 127/220
Chapter Six:
Influence of Fibre Length on Composite Properties
The development of asbestos free fibre cement industry has made it most desirable to
obtain definite information on the relationship between the nature of the natural plant
fibres and their composite prop erties. It has been generally recognized that fibre length
and fibre stiength are two of the most imp ortant factors bu t, because various features of
fibre morphology and chemical composition can influence composite properties, it has
been difficult to obtain a clear picture of the effect of any one property.
Excellent work has been carried out in the field of paper science regarding fibre length and
papermaking properties (Watson, 1961; Page, 1969; Seth, 1990). Apart from improving
sheet formation, which indirectly affects many sheet properties, reducing fibre length has
littie direct influence on the structural and op tical properties of the sheet. The major effect
of decreased fibre length is on the strength properties; most are severely reduced. Thu s,
longer fibres will benefit all tensile properties, tearing resistance and folding enduran ce,
particularly of weakly bonded sheets or sheets wet stiength. How ever, fibre length can be
less important if the sheets are well bonded, because the failure in the sheet can become
controlled by the strength of the fibres.
Various methods have been used for achieving fractionation of fibre length and the se
methods are designed to isolate the effect of fibre length from that of other morphological
and chemical factors. The usual method has been to separate a pulp into fractions of
different fibre length by screening. A mo re direct approach w as used by Brown (1932) in
110
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 128/220
which handsheets, formed with pulps from which the fines had been removed by
screening, were cut mto narrow stiip s by a sharp knife. The shortened fibres so obtained
were reformed into handsheets; these had lower strength properties than the original
handsheets.
Another method by wh ich the effects of fibre leng th variation may b e investigated is to
prepare pulp from individual growth rings. This has been done by Watson for P radiata
(growth rings 2 to 12) (1952) and for
P taeda
(growth rings 2 to
11,
each separated into
late and early wood) (1954). This procedure gave fibres of P radiata ranging from 1.6
mm (growth rings 2) to 3.1 mm (growth ring
11),
and for P taeda from 2.2 mm (growth
ring 2,
late
and early wood) to 3.2 mm (growth ring 11, late wo od) . An increase in fibre
length produced an
hnprovement
in tearing strength but it was difficult to draw any
definite conclusion regarding other strength properties. Variation in cell wall thickness as
is found between early wood and late wood of one growth ring, also had a marked
influence on strength properties.
Watson and
Dads well (1961)
developed another technique by cutting delignified
Araucaria klinkii
chips into different lengths prior to pulping. The sm aller size chip has
more cutting ends thus its pulp has mo re short fibres. Again their work found longer fibre
had much better tear stiength than that of short fibre.
I l l
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 129/220
6 1 Experimental work
6 1 1 F ibre length fractionation work
The fibre was prepared from unbleached high-tear
P radiata
dry
lab
pulp (APM, Maryvale
mill, AustiaUa). The
lab
pulp was soaked in water over-night, disintegrated and Valley
beaten to 550 CS F freeness then subjected to fibre length fractionation.
Various fibre length fractions were prepared using the Bauer-McNett screening method,
guillotined handsheets method and Wiley grinding method.
Weighed beaten pulp was first disintegrated on the British disintegrator for 500 counter
revs and diluted to 0.2% concentration. The diluted pulp (about lOg o.d. weight) was
poured into the Bauer-McNett fibre length classifier, equipped with four series screen
compartments, for 20 minutes (10#, 30#, 50# and 80# US standard screen) (see Appendix
A.2.3). Ideally, the apparatus is designed to produce four length fractions, however in this
study only the fibres remaining on screen 10# and 80# were collected and used in the
composites due to the insufficient separation of fractions by the screen technique. This
procedure was repeated few times until the desired amount of
pulp
was obtained.
Handsheets made from beaten pulp were also used in tiie preparation of cut fibres. Dried
handsheets were cut into 1.0 mm strips with a sharp gu illotine. Strips were soaked in
water over night, disintegrated for 1,000 rev. counts in the British disintegrator and used
for composite fabrication.
112
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 130/220
Air-dried beaten pulp was ground in a Wiley mill to fibre length weighted av. 0.3 mm as
the shortest fraction. Thus five fibre length fractions w ere ready for com posite fabrication
(2 from Bauer-M cNe tt, 1 from guillotine, 1 from Wiley mill and
1
original).
6 1 2 Fabrication and characterisation
The fibre cement composite samples were produced by a slurry / vacuum dewatering and
press technique, followed by air-curing up to 28 days (see section A.3 .2). Each sample
was based on a 130g dry weight of ingredients, ordinary Portland cement was used as
matrix.
Mechanical and physical properties of the composites such as flexural strength (MOR),
fracture toughness, void volume, water absorption and density were determined by the
methods described in section A.4 . The fibre length fractions were measured on a Kajanni
FS-200 fibre length analyser reported as fibre length weighted average.
6.2 Results and discussion
6 2 1 F ractionation of fibre length
A single
pulp
source was separated into 5 length fractions using three methods and these
length fractions are listed in Table 6.1. As mentioned before, the Bau er-M cNe tt apparatus
is designed to produce four length fractions. In our experience, how ever, it was unw ise to
utilise all four fractions due to the fact that diluted pulp fibre has high flexibihty and some
long flexible fibres are able to elude the sm all opening m esh (screen) ba rrier so that there
is a considerable amount of overlap in the four fractions length popu lation distributions as
seen in Figure 6 .1 . Furtherm ore, fractionation not only separates the pulp into fractions of
different length but also effects a separation between fine and coarse fibres.
113
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 131/220
Table 6.1 Fibre length fractions
Length Fraction (mm)
3.13
2.66
1.59
0.74
0.30
Experiment Technique
Bauer-McNett screening
original pulp length
cutfing
dry handsheets
Bauer-McNett screening
Wiley mill grinding
Fig. 6.1. Fibre length population of Bauer-McNett technique four length fraction.
Using a
guillotme
to cut handsheets can reduced fibre leng th to 1.59 mm from its original
2.66 mm . Attempts have been made to further redu ce fibre length by employing a second
cut to the cut fibre formed handsheets, how ever, it was not successful as the second cut
only had very small am ount of reduction in length compared to that from the first cut (e.g.
first cut: 1.59mm and second cut 1.50mm).
Mean fibre length may not provide the best measure of effective fibre length in both paper
sheets and compo site products. The best measurem ent wou ld be the fibre length
distribution. It is quite possible two different p opulation distribution pulps would have
114
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 132/220
same mean arithmetic fibre length ( or even length weighted average). The observation
used in paper science suggests that of two pulps with the same mean fibre length, tiie one
with the greater proportion of long fibres will give tiie higher tearing resistance (Watson,
1961 and CoUey, 1973). The influence of length population distribution to com posite
properties requires further investigation.
6 2 2 Influence of fibre length on com posite mechanical properties
Table 6.2 Relationship betw een fibre length and air-cured com posite pe rformance.
Fibre (w%)
L: 3.13 mm **
2
4
6
8
10
L; 2.66 mm**
2
4
6
8
10
L: 1.59
mm**
2
4
6
8
10
L:
0.74 mm***
2
4
6
8
L: 0.30 mm **
2
4
6
8
MOR (MPa)
18.0+5.1
18.6±3.3
25.9 ±2.3
27.4+2.7
30.4+3.3
18.8+1.1
20.4±1.9
23.4+2.3
24.3±1.9
28.1+3.1
16.2+1.1
18.I+2.I
20.9±1.8
22.3+0.7
22.6±2.3
13.3±0.7
17.6+3.7
22.4+0.9
23.6+2.5
13.6+1.0
143±0.5
14.1+0.7
14.3+1.3
Frac. Tough(kJ/m2)
0.33+0.03
1.07±0.34
1.68±0.28
2.39±0.40
2.89±0.6I
0.38+0.10
0,87±0.12
1.35+0.24
1.87±0.19
2.80+0.45
0.22+0.02
0.59±0.07
0.89+0.11
1.37±0.17
1.68±0.28
0.24±0.03
0.58±0.19
0.91±0.10
1.23±0.27
0.07±0.01
0.09+0.01
0.13±0.02
0.18±0.01
Void Vol (%)
33.3±0.7
33.6±1.0
33.0±0.9
34.6±0.4
35.0±0.8
29.2+0.4
30.5±0.3
32.6±0.9
33.4±1.2
34.6±1.0
29.5±1.0
29.4±0.8
30.6±0.6
32.4+0.6
34.3+0.8
32.9+0.5
34.2±0.4
35.4±0.7
36.7±0.6
28.9±0.6
29.9±1.0
31.8±0.8
33.3±0.5
Water Abs.(%)
18.4±0.6
19.7±0.6
20.3±1.0
22.3±0.4
23.3±0.8
15.64+0.3
17.46+0.3
19.84+0.8
21.57+0.8
22.47+1.1
15.9±0.7
16.8±0.6
18.1±0.3
20.2±0.6
21.9+0.6
18.3+0.4
20.5±0.4
22.5±0.8
23.1±0.6
15.2±0.4
15.5±0.7
18.4±0.6
20.5±0.6
Density (g/cm')
1.81±0.02
1.70±0.03
1.63±0.03
1.55±0.01
1.50±0.02
1.86±0.02
1.75±0.02
1.65+0.03
1.55±0.05
1.54±0.03
1.86±0.02
I.78±0.02
1.70±0.05
1.60±0.02
1.57±0.06
1.80±0.01
1.67±0.01
1.58±0.02
1.55±0.02
1.90+0.01
1.82+0.02
1.72±0.02
1.62±0.02
* Pulps of fibre length w eighted a verage
3.13
mm and 0.74 mm w ere generated from Bauer-McN ett classifier screens; pulp of fibre
length weighted average 1.59 mm was prepared from gu illotined dry handsheets and pulp of fibre length we ighted average 0.30 mm
was made from W iley mill. The initial pulp for this study was Freeness 550 CS F, length weighted average 2.66 m m and kappa number
around 30 P .
r di t
Kraft pulp.
** 3 standard deviation, sample size n=9. *** 3 standard deviation, sample size n=6.
The results of effective of fibre length to air-cured com posite mechanical and physical
properties are shown in Table 6.2 and Figure 6.2 to
6 6
Fibre lengths between 0.3 mm to
115
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 133/220
3.13
mm w ere used at weight content between 2 per cent to
10
per cent and the
comparison of properties was made at 28 days.
3 5
3 0
» 5
a
'2 15
X
u 10
4 6 8
Fibre Co nto rt ( by mass)
10
12
Fig.
6.2.
Effect of fibre content on composite flexural strength for different fibre lengths.
Fig. 6.2 shows the effect of fibre content on flexural
sU-ength
for different fibre lengths.
Except the very sho rt length fraction (L: 0.3m m), the highe r strength of the composites
with longer fibre fractions can be attributed to the increased length efficiency factor (see
section 2.1.1). The longer fibre fraction had better strength property. The length
contribution however was not that significant to the strengtii improvem ent. For exam ple
at 8% fibre content, witii length fibres
neariy
doubled (1.59 mm and 3.13 mm) the
composites strength values increased from 22.3 MPa to 27.4 Mpa, respectively.
The porosity of the com posite increases as tiie fibre content increases and fibre length
increases (see Tab le 6.2). This effect m ay be the cause of the reduction in flexural
stiength of som e of tiie composites at fibre content g reater than 8 to 10 per cent by w eight.
116
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 134/220
Q
5
Z
D>
C
(1)
—
W
S
u
3
l
x
LL
30
25
20
15
10
5
0
_
»
^_—
^• ^
/ •
^
^^^' ^^^^
^-^ ^''^
• • —
' - - - • • , , ,, , , , ,, , , „ , , „
0.5 1 1.6 2 2.5 3 3.5
Fibre Length Weighted Av. (mm)
Fig. 6.3. Influence of fibre length on composite flexural strength at
8%
fibre content
Again Figure 6.3 shows composite strength increased with fibre length increased at 8 %
fibre by mass, although tiiere were some small strength variations to composites made
from fibre leng th 1.59 mm and 0.74 mm . This might be explained by the fact that mean
fibre length may not be the best measure of effective fibre length. The 1.59 mm length
fraction m ight have a greater percen tage of fines than the 0.74 mm fraction (due to
different fractionation techniques) and such fines do not contribute to com posite strength
development ( see results of the 0.3 mm fraction). Density results in Table 6.2 and Figure
6.7 seen to support the above argument as composites made from the 1.59 mm fraction are
denser than those from the 0.74 mm fraction.
Fibre length contributes greatly to com posite fracture toughness development. Over the
whole range of fibre contents, the longer fibres have higher values of composite toughness
(Fig. 6.4 and 6.5). The length contribution to the fracture toughness is much greater than
that to the flexural strength.
117
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 135/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 136/220
the matrix, after the fibre to matrix bond is broken and the greater the contribution to the
dissipation of energy contained in the advancing crack.
The same proposition was developed by the
histitute
of Paper Chemistry (Staff of IPC,
1944) on the studies of tiie influence of fibre strength to paper tearing resistance. The
tearing resistance is the sum of the work done in breaking some fibres and the work done
in pulling the remainder out of the fibre mat. The maximum tear is obtained at a degree of
interfibre bonding such that the greatest number of fibres required a force just short of
their breaking load to pull them free, while the level of the maximum depended on fibre
length as the parameter controlling the distance over which that force is applied.
Studies of synthetic fibres such as steel, polyproylene, glass and more recentiy kevlar and
carbon reinforced cement based composites led to the same conclusion that composites
reinforced with long fibre have better strength and toughness properties. The area under a
3-point bend load-deflection curve is often described as a measure of the toughness or
energy absorbing capability of the material. Various values for toughness can be
calculated for the same curve depending upon whether the complete load-deflection curve
is used including the descending portion or whether a cut-off point is chosen. From the
point of view of serviceability of a structural unit a mo re meaningful value for toughness
can be obtained from the area up to the maximum load or up to a specified deflection
depending on the degree of cracking allowed m service. Hannant (1978) in his book
Fibre
ement and Fibre oncretes
reported that Johnston
(1975) surveyed the data for the
complete load-deflection curve and also for the area up to the maximum stress (Fig. 6.6).
It can be seen that the ab ility of a composite unit to absorb energy is substantial and
119
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 137/220
increases with fibre content and fibre length (or with length and width aspect ratio), even if
the cut off point is taken at the maximum stress.
Area under complete
load deflection curve
Area under
load deflectioo
curve up to maximum stress
2 4 6
WL d
8
100O
Fig.
6.6.
Influence of fibre WL / d on composite fracture toughness. Where W =
fibre
content by weight x
100,
L =
fibre
ength,
d =
fibre
diameter (Hannant, 1978).
The interesting finding in this work is that very short fibre (fragments) say, with length
less than 0.3 mm, act more as a filler-diluent then as a reinforcing fibre when used to make
composite ma terials. So a greater mass of pulp is needed to have sufficient num ber of
long fibres in the high proportion short fibre pulp reinforced composites such as bamboo
fibre reinforced cemen t and waste paper fibre reinforced products (C outts,
1994a
and
1989).
Due to the variation of wood fibre diameter (early wood and late wood) and the lack of
fibre diameter (coarseness) values no attempt is made to determine fibre aspect ratio
influence. Furthe r, no definite critical fibre leng th value has been concluded in this study
because there is a wide length gap betw een 0.3 mm and 0.74 mm . Further w ork is needed
120
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 138/220
to resolve tiiese unansw ered questions, however, the general picture of the effect of fibre
length is obvious.
6 3
Theoretical and experim ental conflict in results
We have discussed early in section 2.1 that the composite strength can be calculated from
the equation (2.4) when the fibre content is less than 8% by mass.
a,^ = [a/p] cT„,.iv,.,., +
0.82 aT)vf l/d)
(2.4)
Where the a, fi, (J„,h are constants which can be determined from experiments
(Andonian, 1979). Furthermore, the fibre diameter d is suggested to be constant for a
certain type of fibre. Thus at a constant fibre con tent
v/
and v.,,, the composite strength
increases lineally w ith increasing fibre leng th [equation (6.1)].
a, = Kj(l) K2 (6.1)
Where Ki and K2 are constants.
However, the experimental results from this study do not support the theoretical equation
(6.1). A simple regression analysis of the data presented m T able 6.2 leads to a relation
(6.2).
Gh = 3.3 1)
16.9
(at 8% fibre conten t) (6.2)
The relationship shown in equation (6.2) is statistically unacceptable suice the statistical
multiple coefficient
(R^)
is only about 0.66 which is very low compared to the theoretical
estimation of approximately 0.99.
121
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 139/220
The
non-conformability
of experimental results with the underlying theory could be
attributed to the theoretical assum ptions made. The fibres are pulled out of the cement
matrix instead of broken at failure. And , the fibres hav e uniform diameter, the fibre-
matrix interfacial bond strength is constant. The natural fibre, however, violates these
assumptions.
The fracture mechanics concept as discussed in Chapter 2 can not be used to correlate the
experimental results obtained in this study. If LEF M has to be valid, the stress intensity
factor,
K;;
in general and the fractural toughness,
Ki,-in
particular have to be defined in
terms of crack geometry and crack location which can not be ascertained for the kind of
composites used in this study, ie., the Ki,,
is difficult to establish based on standard Kic test
procedure.
6.2.4 Influence of fibre length on composite physical properties
The changing of fibre length and propo rtions of the constituent fibres and matrix affect
void volum e, density and water absorption (Table 6.2). Short fibre (0.3 mm ) results in
close packing of ingredients, thus the composites are more dense than those made with
long fibre as showed on Figure 6 .7. In general, long fibres cause formation and running
abihty problems m paper making and have
"baUing
up and drainage problems in cement
products. However, the fact these problem s did not appear might be due to greater control
of the sample preparation at the laboratory level. There were some density variation for
composites made from fibre length 0.74 mm and 1.59 mm fraction. This variation had
caused composite strength inconsistency, tiie reason for this could be explained by the
fibre length population distribution.
122
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 140/220
Water absorption and void volume change witii density changes in keeping with the eariy
results. If the composite is mo re dense, the amount of porosity in the composite and hence
water absorption and void volume values are lower.
1 7
1 4
0 3
0 74 1 59 2 66
Fibre Length
sightad
Av. mm)
3 13
Fig. 6.7. Influence of fibre length on com posite density.
6.3 Conclusions
A single
P radiata
Kraft pu lp has been divided into 5 different length fractions varying
from 0.3 mm to 3.13 mm . Fibre length makes a major contribution to tiie mechanical and
physical properties of air-cured cement compo sites. Both comp osite flexural strength and
fracture toughness values increase with increasing fibre length over the range of fibre
contents examined. The fracture toughness can be significantiy improved w ith long fibres.
The very short fibres (fragments) say, with length less than 0.3 mm, act mo re as a filler-
diluent then as a reinforcing material when used to make composite materials.
123
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 141/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 142/220
Chapter Seven:
Influence of Fibre Strength on Composites Properties
In section 2.1.1 we discussed the rule of mix ture with respect to composite strength
which suggests that stronger fibre and higher fibre loadings lead to greater composite
strength. The influence of fibre content has been studied by a num ber of peop le and the
results suggest an optimum fibre content of about 8 - 10% by weight for softwood fibre
(Coutts, 1979a and AU Patent 1981). As well as fibre content, there is a need to study
natural fibre strength as a parameter affecting composite performance.
Lhoneux and Avella (1992) plotted a wide range of wood pulp fibre tensile strengths
against the fibre-cement composites flexural strengths. A nearly linear relationship was
observed and they suggested the stronger fibres provided better reinforcement than the
weaker fibre. The results seemed in agreem ent with the theory of mixtu re rule; however,
their work ignored the influence of other fibre factors which might also have impact on
composite strength. For exam ple, they ignored the influence of fibre length between the
softwood and hardwood; ignored the influence of fibre chemical composition between the
chemical pulp and mechanical pulp fibres; ignored the influence of fibre fibril angle and
cross-dimension between different species.
The objective of this chapter is to investigate the influence of fibre strength on fibre-
cement compo site properties. Therefore, the experimen t is designed to vary the fibre
strength while maintaining constant as many fibre parameters as possible.
125
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 143/220
7.1 Exp erimental work
7 1 1 Fibre strength fractionation
As discussed before the key point of the
experunent
is to fractionate fibre intrinsic strength
without changing other parameters (eg. fibre length and freeness). This could be achieved
by means of cellulose degrad ation. Wood cellulose fibre can be reduced in strength by
alkaline, oxidative, hydrolytic and thermal methods or combinations of these conditions
which result in degrada tion reactions of cellulose. There are two metho ds used in this
study to prepare degraded fibre, namely extensive alkaline cooking and liquid phase acid
hydrolysis.
7.1.1.1 Extensive
ahcahne
cooking method
The fibre used in this me thod was holocellulose fibre, which was prepared from Kraft
pulped P radiata
(-2.6%
lignin content) by chlorite dehgninification method (see
Appendix A .
1.4).
After dehgnification, the fibres were subjected to further alkaline
(Kraft) cooking to produce fibres of various strengths. Th e details of cooking conditions
and fibre strength, length and freeness values are reported in Tab le 7. 1.
7.1.1.2 Liquid phase acid hydrolysis method.
Liquid phase acid hydrolysis method was apphed to
P radiata
Kraft pulp (-2.6% lignin).
Each lOOg of pulp was slushed m 7 litre water. 1 litre hydrochloric acid (HCl), lOM was
added w ith stirring. The fibre was attacked by the acid for variou s periods of time , such as
50h, 70h, 140h and 528h. After each desired period, the pulp w as filtered and washed free
of acid.
126
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 144/220
7 1 2 Fibre quality evaluation
Fibre strength was evaluated as Zero-span wet (unbonded)
tenstie
index
(ZSTI)
and the
degree of polymerisation (DP). ZSTI was conducted on Pulmac Zero-span Tensile Tester.
It is a standard measurem ent used in pulp and paper science to evaluate pulp fibre tensile
strength (see Append ix A.2.5.2). Ho locellulose fibre polym ers are subjected to
degradation during further alkahne cooking proce ss. This degradation results in fibre
strength loss and can be evaluated by means of the degree of polymerisation (DP ). DP
was determined using 0.5M cupriethylenediamine as a solvent and a capillary viscometer
in this study (see Appendix A.2.5.1).
The fibre weighted average length and freeness value was measured on Kajaani FS-200
Fibre Length Analyser and Canadian Standard Freeness Tester, respectively(see Appendix
A.2.3 and A.2.2). Fibre length and freeness are two important parameters associated w ith
reinforced composites (see Chapter 6; Coutts, 1982). It was desirable to maintain the
constant length and freeness while the strength was altered by chemical treatment.
7 1 3 Com posite fabrication and ev aluation
Composite specimen were produced by a slurry / vacuum dewatering and pressing
technique with 8% (by mass) sttength modified fibres as the method described in
Appendix A.3. Both air-cured and autoclaved samples were made. In order to minim ise
the matrix and curing effect, composites reinforced with different strength fibres we re
cured at the same time to eliminate variations in the curing regime.
27
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 145/220
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 146/220
Table 7.1 Prop erties
of P radiata
fibre after alkahn e cooking
Further cooking condition
ZSTI (Nm/g) Viscosity (mPa.s) W. Length (mm) Freeness (CSF)
holocellulose untreated
E.A13%
30 mins
to
140°C
E.A26% 30 mins
to
KCC
E.A26% 60min to.lOmin
on
180°C
100
89.1
81.2
44.0
16.27
12.96
13.66
6.50
2.48
2.32
2.11
2.18
680
680
700
700
20
40
60 80 100
Fibre Zero-span Tensile Index Nm g-1)
120
Fig. 7.1.
Relationship between fibre zero-span tensile strength and pulp viscosity
3.5
20
4 0 60 80 100
Fibre Zero-span Tensile Index Nm
g-1)
120
Fig. 7.2. Relationship between fibre strength
and
fibre length
129
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 147/220
900
400
20
40 60 80 100
Fibre Zero-span Tensile Index Nm g-1)
120
Fig.
7.3.
Relationship between
fibre
strength
and pulp
freeness
Holocellulose fibre was treated with extensive alkaline cooking to produce pulps of
various stiength values. There were two reasons to use holocellulose fibre. Firstiy these
hgnin-free fibres allowed further treatment to various fibre strengths without the
complication of products w ith different lignin levels, which w ould affect the fibre strength
determinations and composite fabrication. Secondly holocellulose fibre was found easier
to subject to further alkaline degradation than any other mechanical or chemical pulped
fibre. This may be related to the larger quantity of effective alkah (on a sample basis) in
the holocellulose co oking, or to the influence of lignin on the pulping process or to the
consumption of alkali (McKenzie, 1985).
Due to limited pulp source, the properties of pulp treated with hydrochloric acid were not
determined. Howev er, the evidence of fibre stiength loss caused by acid attack was
observed by a number of researchers (McK enzie, 1985 and Guma gul, 1992). Gum agul
(1992) suggested that there were two types of degradation processes which control the
extent of the strength loss. Homogeneous and random degradation causes littie strength
130
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 148/220
loss, but locahsed degradation w eakens fibre significantiy. The above two treahnents
would possibly cause localised degradation resulting in a significant loss of fibre strength.
7 2 2 Influence of fibre strength on composites properties
7.2.2.1 Alkaline treated holocellulose fibre
Table 7.2 and Figure 7.4, 7.5 shows the variation of composite flexural strength when
reinforced with fibres possessing a wide range of strength values. It is surprising that the
composites flexural strength did not dec rease significantly as fibre strength reduced from
ZSTI 100 Nm/g to ZSTI 44 Nm/g in both air-curd and autoclaved WFR C specun en. The
strength of the specimens reinforced with very weak fibre (ZSTI 44 Nm /g) started to drop
off, but at the same time tha t point the fibre length was somewha t shorter w hich could
have an effect on the strength. This observation was inconsistent with the rule of mixture
of composites strength development.
Table 7.2 Com posites properties reinforced w ith alkaline treated holocellulose fibres
ZSTI (Nm/g)
Air-cured WFRC
100
89.1
81.2
44.0
Autoclaved WFRC
100
89.1
81.2
44.0
MOR (MPa)
25.1+1.3
24.0±2.2
25.611.3
21.4±1.5
19.9+2.9
21.4+1.7
21.1+1.3
18.6±1.2
Tough (kJ/m2)
2.40+0.39
1.90±0.28
1.42±0.26
0.9010.14
1.6210.51
1.4610.44
0.9210.19
0.5110.12
Void Vol
(%)
32.9+1.0
33.310.2
32.310.9
31.7+0.5
42.510.8
41.910.5
41.8+1.2
42.110.8
Water
Abs.(%)
20.711.1
21.110.1
19.911.0
19.210.6
30.911.2
30.4+0.7
30.311.5
30.511.0
Density
(g/cm-*)
1.5910.03
1.5810.01
1.62+0.03
1.6510.02
1.3710.03
1.3810.02
1.3810.03
1.38+0.02
* 3 standard deviation, sam ple size n=9.
131
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 149/220
Q
0 .
5
x:
O)
c
w
1-1
re
3
X
V
LL
3b r
30
25
20
15
10
5
•
D
D Autoclaved WFRC
• Air-cure d WFRC
•
n
n
-
o
20
40 60 80
Fibre Zero-span Tensile Index Nm g-1)
100 120
Fig. 7.4. Relationship between fibre strength (0-span) and composite strength
35
? 30
a.
25
o>
2 0
ID
1 10
X
LL 5
D Autoclaved WFRC
• Air-cured WFRC
_a_a
10 12
Viscosity mPs.s)
14
16
18 20
Fig. 7.5. Relationship between fibre strength (viscosity) and composite strength
The composite fracture toughness values decreased greatiy when the reinforcing pulp
contained weak fibres (Figure
1 6
and 7.7). At a fibre oading of 8% by mass and fibre
strengtii decreasing from ZSTI 100 Nm/g to ZSTI 44 Nm/g, the composite fracture
toughness values were reduced from 2.40 kJ/m^ to 0.90 kJ/ for air-cured samples (about
62%
reduction) and from 1.62 kJ/m^ to 0.51 kJ/m^ (about
69%
reduction) for autoclaved
samples, respectively. This result suggests that the weak fibres are not suitable as a
132
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 150/220
reinforcement for cement products, due to the composites lack of essential fracture
toughness, even when having acceptable strength values.
2.8
CM
I
E
/>
0)
c
£
O
3
O
U
TO
w
U.
2.4
2
1.6 I-
1.2
0.8
0.4
0
o Autoc laved WFRC
• Air-cured WFRC
20
- e -
40 60 80
Fibre Zero-span Tensile Index Nm g-11
100
120
Fig. 7.6. Relationship between fibre strength (0-span) and composite fracture toughness
_ 2.8
E 2.4
-
^ 2
v
g 1 6
t 1.2
o
I -
f l ) 0 .8
i 0.4
°
Autoclaved WFRC
• Air-cured WFRC
10 12
Viscosity mPs.s)
14 16 18 20
Fig. 7.7. Relationship between fibre strength (viscosity) and composite fracture toughness
The mechanisms that take place when a cement based fibre composite is loaded to failure
include fibre fracture and fibre pull-out. The fracture toughness value is determined by a
combination of these two mechanisms. If the fibre is weak, then the energy required to
fracture the fibre is less than that to break the fibre-matrix bonds plus pulling the fibre out
133
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 151/220
of the matrix. Thus the weak fibre is likely to break before being pulled out and the
composite is brittle.
The above explanation of fracture toughness was
confkmed
by examination of the
composite fracture surface using scanning electron microscopy. When composites were
reinforced with strong fibres, fibres were both fractured and pulled out durmg the fracture
(Figure 7.8) and the specimen had an irregular fracture surface and high fracture toughness
values resulted. Altem atively, composites reinforced with weak fibres showed a fracture
surface that was flat, most fibres broke at the crack front and the crack could continue
right through the matrix. The com posite showed brittle behaviour and had low values of
fracture toughness (Figure 7.9). In this study other factors such as fibre-matrix bond,
chemical composition and fibre morphology were all constant, so the difference of
composite properties were m ainly caused by the difference of reinforcing fibre strength.
Fig. 7.8. Fracture surface of
composite
reinforced with strong
fibre
shows
fibre
pull-out.
134
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 152/220
Fig. 7.9. Fracture surface of composite reinforced with weak fibre shows fibre fracture.
By contrast it is difficult to explain the behaviour of composite strength. It could be
suggested, however, that for the high modulus cement based composites even a
low
percentage of
low
modulus fibres has an initial crack-stopping effect, but for further
strength developm ent a strong fibre and good fibre-matrix bond is required. In the current
study the fibre strength (even ZSTI 100 Nm/g fibre) or fibre-matrix bonding might not be
sufficient enough to produ ce further strength increasing. Thus the composites showed an
initial strength which was similar.
The constant density, water absorption and void volume of composites were expected
from the same matrix, fibre content and morph ology . The slightly higher value of density
and lower value of void volume and water absorption for the composite reinforced with
weak fibre maybe due to the conformability which was caused by heavy degradation.
135
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 153/220
7.2.2.2 Liquid phase hydrochloric acid (HCl) treated fibre
The study of composites reinforced with hydrochloric acid treated fibre provided the same
conclusion that weak fibres have littie influence on the composite strength but can
significantly reduce tiie composite toughness (see Table 7.3 and Figure 7.10, 7.11). In this
instance, cellulose degradation was not as easy as that for the holocellulose. The results
showed that both composite stiength and toughness values were not greatiy affected
although the fibre had been attacked by acid for more than 140 hou rs. But after 528 hou rs
treatment, composite strength and toughness were reduced from 22.3 MPa to 19.1 MPa
and from 0.79
kJ/mHo
0.39
kJ/m',
loss about 14% and
51%),
respectively.
Extensive discussion in the next chapter will show that the same source of fibre pulped to
different Kappanumber (lignin level) can cause composites to have various toughness
values (see Chapter 8). This study will
help
us to better understand the mechanics of the
composite fracture, becau se different Kappa number pulp has been strongly associated
with its fibre strength (Page, 1985).
Table 7.3 Composites properties reinforced with acid hydrolysis fibres
Fibres
Un-treated
50 hours treated
70 hours treated
140 hours treated
528 hours treated
MOR (MPa)
22.311.8
21.7+2.3
20.811.6
20.911.6
19.1+1.4
Tough (kJ/m2)
0.7910.08
0.77+0.09
0.8710.10
0.77+0.13
0.39+0.03
Void Vol
(%)
30.611.2
31.610.6
28.9+0.7
28.710.9
26.1+1.3
Water Abs.(%)
42.5+1.0
42.710.5
40.210.6
40.211.2
37.811.2
Density (g/cmr)
1.3910.02
1.3610.06
1.3910.01
1.40+0.02
1.4510.03
* 3 standard deviation, sample siz e n=9.
136
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 154/220
1
2 3 4
Acid Hydrolysis Time hour)
5 6
Fig. 7.10. Relationship between fibre acid treated time and composite strength
1 £
1
•
1 0.6
3
Q
•
1 0.4
0.2
'
-
-
^ ^ ~ ^ ~ ^ ^ - ^
^ ^ ^ ^ ^ ^ - . ^ ^
^ ^ ^ ~ \
,
r _ _ _ l
1
2 3 4
Acid Hydrolysis Time hour)
5 6
Fig.
7.11.
Relationship between fibre acid treated time and composite fracture toughness
Coutts studied composites reinforced with NZ flax fibre(Coutts, 1983c). The composites
showed similar flexural strengtii values, but, lower fracture toughness values compared to
137
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 155/220
the com posite reinforced w ith softwood fibre (Cou tts, 1984a). This was possible due to
the fact tiie strength of NZ flax Kraft pulp fibre was weaker than that of softwood fibre.
In addition, Stevens
(1992)
reported that autoclaving cycle had a significant effect on
composite toughness. In his work the first autoclaving cycle produced specimens which
were quite tough. How ever, after ten
autoclavuig
cycles the samples were
exhemely
brittie. The results could be explained by the fact that cellulose degraded and the fibre
strength reduced gradually after each autoclaving cycle. These weakened fibres w ere not
able to provide the composite w ith sufficient
fracmre
toughness.
7.3 Conclusions
Holocellulose and Kraft wood pulp fibres were chemically treated to various fibre tensile
strength without chang ing other parameters. The results suggested that the fibre strength
was particularly important to composite toughness but not as important to the strength. In
both air-cured and autoclaved cases, when fibre strength was reduced to about 40%, the
composites had a toughness loss more than 60%, but the strength was virtually unchanged.
Examination of SEM micrographs, of the composite fracture surfaces, showed that the
samples containing the weaker fibre had produced the expected higher population of
broken fibres than samples contaming the stronger fibres.
138
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 156/220
Chapter Eight
Influence of Fibre Lignin Content on Composites Properties
Durmg the course of this stiidy, we found that pulps with different Kappa numbers (made
from the same wood chip source) could result in a significant variation of composite
properties, particularly values of fracture toughness. This finding suggested further
investigation of the influence of lignm content (Kappa number) on the com posite
properties was warranted.
Along with cellulose and hemicelluloses, lignin is another major chemical component of
plants. It is presen t to the extent of about 24 to 32% in softwood, lower amount in non-
wood plants and very little in cotton and bast fibres (see section 3.3). The influence of
lignin content on pulp and paper properties has been studied by several researchers
(Gieriz, 1961; Page, 1985), despite the extremely complex nature of hgnin chemistry. In
general, in a non-degrading pulping process, fibre strength improves with decreasing
lignin content due to the higher cellulose content. Pape r formed with these strong fibres
has better mechanical prope rties. Howev er, further reduction in lignin content such as
pulping to a yield corresponding to an
a-cellulose
content higher than 80% or bleaching
tends to reduce fibre strength, apparently due to the elitnination of this stress-equalising
matrix hgnin or due to cellulose degradation. Furthe rmore, because of the hydrophobic
character of lignin, it binds the fibrils firmly together (fibres become stiffer) even after
wetting. Lignin also decreases the interfiber bonding in paper.
139
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 157/220
Information regarding the influence of hgnin content on composites is very hmited. Mai
and co-workers (1983) studied low hgnin content bleached pulp and Kraft pulp autoclaved
com posites. The com posites were reinforced with commercial Wisakraft bleached pulp
and Kinleith Kraft pulp and fabricated by the Hatschek process. They found that bleached
pulp composites had higher flexural stiength but lower fracture toughness values than
those of unbleached Kraft pulp.
Coutts (1986) studied high lignin content TMP and CTMP pulps prepared from
P radiata
chips as reinforcement for autoclaved and air-cured composites (fabricated by slurry /
vacuum de-watering technique). Both pulps were unsatisfactory, for use as a fibre
reinforcement alternative to Kraft pulp in the autoclaved cement mortars, but some gave
acceptable air-cured p roducts. It was suggested that during autoclaving the extracted
chemicals (eg. lignin) formed could cause an inhibiting effect on the curing of the cement
and the deposit of such products could effect fibre-matrix bond ing.
The fibre-cement industry has already committed itself to the use of Kraft softwood pulp
as the asbestos alternative source in Australia. The level of pulping yield (lignin content)
is not identified, but Kraft pulp at kappa number around 25 (lignin content about 3.7%) is
well suited for autoclaved WFRC products.
Base on the above data, it was considered appropriate to study the relationship between
fibre lignin content and fibre cement composite properties.
140
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 158/220
8 1 Experimental work
8 1 1 Fibr e p reparation
This study was designed to investigate the effect that different hgnin containing pulps had
on reinforced fibre cement composite properties. The pulps studied contained various
levels of Hgnin and included a thermomechanical pulp (TMP), a chemithermomechanical
pulp
(CTM P), three Kraft pulps and an oxygen delignification pulp. The pulps were
prepared from selected P radiata chips (APM, M aryvale mill, Austraha) in the CSIRO ,
Division of Fore st Produ cts pulping laboratory. The detailed pulping techniques have
been discussed in Appendix
A.
1. W hile, a brief description of these techniques is listed in
Table 8.1.
Table 8.1 Pulping techniques
Pulp Pulping Technique
TMP presteam chips 1 -2 min at
120-
125°C, followed by Asplund defibrillator 2-3 min,
then
refined
in Bauer
refiner
CTMP presoak chips in 10% casustic soda over 18 hours, then processed as in the case
of TMP pulp
Kraft (Kappa
No.44.56)
E.A. 11.51% (NaOH), 4:1 liquor
to wood
ratio,
2 h
to 170°C and
2 h
at
170°C
in
air-bath (3L pulping vessels)
Kraft (Kappa No.27.49) E.A. 13.56% (NaOH), 4:1 liquor
to wood
ratio,
2 h
to 170°C
and 2 h
at
170°C in
air-bath (3L pulping vessels)
Kraft
(Kappa No.17.33)
E.A. 14.00% (NaOH),
4:1
liquor
to wood
ratio,
2 h
to 170°C
and 2 h
at
170°C
in
air-bath (3L pulping vessels)
Bleached Kraft pulp (E.A.16.63%)
mixed with 1%
MgC03,
2%
NaOH
at
10%
consistency
cooked with O2 (780kpa), 75 min to 115°C and 30 min at 115°C
All pulps were tightly beaten in a Valley beater at condition of each 360g (o.d.) with 23
htre water without load for 20 min then with 1.5 kg bed-plate load for another 20 min.
The beating effect was evaluated by means of Canadian Standard Freeness tester (see
Appendix A.2.2).
141
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 159/220
8 1 2 Fib re lignin content an d other properties evaluation
The pulps prepared from above techniques cover a wide range of lignin contents and there
is no single lignin evaluation m ethod available. For high yield pulp such as TM P and
CTM P pulps , Klason Hgnin method is more suitable, whUe for Kraft and oxygen bleached
pulps. Kappa num ber test is normatiy applied. The percentage of lignin present is then
estimated as 0.147 X Kappa number value. Because softwood contains only a small
percentage of soluble lignin, there was no attempt to correct for soluble hgnin in Klason
Hgnin measurem ent. Th e detailed lignin content evaluation methods should be refereed to
in Appendix
A.2.1.
Other fibre prope rties such as strength, length and freeness values are measured as wet
zero-span tensile index, Kajanni length weighted average and Canada Freeness
respectively. The testing method s are described in Appendix A.2 .
8 1 3 Com posite fabrication and characterisation
Air-cured and autoclaved composites were fabricated and characterised by the methods
described in Appendix A.3 and A.4.
8.2 Results and discussion
8 2 1 Fibre l ignin conten t and oth er pro pert ie s
Table 8.2 Fibre Hgnin content, freeness and fibre strength, fibre length
Pulp Lignin (%)/metiiod Lengtii (mm) Freeness (CSF) Strengtii as ZSTI
TMP
25.52/Klason
2.17 700 low
CTMP 26.04/Klason 2.43 730 low
Kraft
44.56*
6.47/Kappa 2.53 730 109
Kraft 27.49 4.04/Kappa 2.60 700 113
Kraft 17.33 2.55/Kappa 2.36 670 101
Bleached 0.69 / Kappa
2 29
670 98
* The number after Kraft is its Kappa number
142
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 160/220
The variation of pulp Hgnin content, freeness, fibre strength and fibre length are
summ arised in Tab le 8.2. As men tioned before due to the softwood only containing a
small percentage of soluble lignin, there was no attempt to correct for such soluble Hgnin
in Klason lignin measurement. There was an inconsistency between TM P and CTM P
Hgnin content as tiie values of 25.52 for TMP and 26.04 for CTMP could be
expected to be reversed. Klason lignin measures insoluble lignin on pulp base, CTMP
pulp could show a higher lignin content than that in the same amount of TMP pulp due to
pretreatment of caustic soda extracting other soluble ma terials from the fibres. The
principle of mechanical pulping is to mechanically separate wood into its constituent
fibres. M ild chemica l treatment during CTM P pulping would not cause much lignin loss,
thus TMP and CTMP could be expected to have similar Hgnm content as that of the wood.
However, the difference in other properties of these pulps would have som e influence on
the final composite performance.
Although it would be desirable to maintain a constant length, so as to limit the variables
when studying Hgnin effect, som e variation was recorded. It would not be expected to
have a major influence (see Chapter 6).
In addition, the difference of fibre strength could also have some effect on composite
performance particulariy the fracture toughness as studied ui Chapter 8. The pulp with
Kappa number around 27 had the highest strength index of the six pulps. The number of
un-degraded cellulose fibres in the pulp makes the major contribution to the strength
development of the pulp. Besides cellulose content, mechanical pulps (TMP and CT MP )
and high Kappa number pulp (Kraft 44) contain a high percentage of lignin and
143
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 161/220
hem icellulose . There are less ceUulose fibres present in the high yield
pulp
compared to
the low yield pulp (e.g. Kraft 27), thus a lower strength mdex is expec ted. The decrease
in strength of tiie pulps with the Kappa num ber below 27 such as Kraft 17.33 and bleached
pulp could be due to degradation of cellulose from strong alkah cooking or bleaching.
The high freeness value suggested that 20 min, with low bed-plate load.
Valley beating
would probably only improve fibre
conformabihty rather than fibriUating fibre surface.
Although the TMP and CTMP pulps contained high fines proportion, they
stiU
showed
relative high freeness values because of the stiff nature of the fibres.
8.2.2 Influence of lignin content on air cured composites
The influence of fibre hg nin content on air-cured com posite flexural strength, fracture
toughness and density are
dehneated in Figure 8.1 , 8.2 and 8.3, respectively. Th e detailed
results is also listed in Table 8.3.
Figure 8.1 shows the flexural strength variation as fibre con tent was increased from 1 to
12%
by mass for six different lignin co ntent fibre-cement composites. The strength of the
composites start to decrease for fibre content over 10 % due to poor fibre distribution
throughout the matrix material. This observation was in general agreement with the
change in flexural strength observed with early studies in WFRC and other NFRC
(Coutts,1985, 1987b, 1994b).
144
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 162/220
- c ^ *
1 1 1
•
^ — ^
. r S S ^ ^ ^ *
1
• - ^ • ^
X TMP
^
CTMP
O K44
•
K27
- K17
+ Bleached
6 S
Fibre Content | by mass)
10 12
14
Fig. 8.1. Infiuence of fibre lignin content on air-cured W FRC flexural strength at different fibre content
6 8
Fibre Content
(
by mass)
10
12
Fig.8.2.
Influence of fibre lignin content on air-cured W FRC
fractiire
toughness at different fibre content
145
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 163/220
Table 8.3 Influence of fibre Hgnin content on air-cured WFRC.
Fibre
(w%)
TMP, 25.52%
2
4
6
8
10
12
CTMP, 26.04%
2
4
6
8
10
12
K44,
6.47%
2
4
6
8
10
12
K27, 4.04%
2
4
6
8
10
12
K17, 2.55%
2
4
6
8
10
12
Bleached, 0.69%
2
4
6
8
10
12
MOR (MPa)
16.4±1.4
17.4±2.0
19.712.5
23.6±2.5
23.6±2.6
23.3±3.3
15.111.7
15.912.2
20.312.7
21.2+3.4
23.314.6
20.413.5
16.011.6
21.211.4
22.712.3
22.612.8
24.412,2
25.111.2
18.811.1
20.411.9
23.412.3
24.311,9
28,113.5
26,515,4
15.310.9
19,810.9
22,612.9
22,612.5
24.912,3
27,211,4
14,211.1
20.811,5
22.9+2,5
25,014,1
26,612,4
27,114,2
Frac, Tough(kJ/m2)
0,15+0,02
0.35+0,07
0.5210,06
0,7710,13
1,0410,17
1,2110,24
0.2110,05
0,4310,07
0,7910,16
1,0310,29
1.3410.34
1,2910,26
0,4810,06
0,9210,12
1.4510,20
1,7810,28
2.4310,67
2,4810,09
0,3810,10
0,8710,12
1,35+0,24
1,8710,19
2,8010.45
2,7710.67
0,3410,03
0,7010.10
0,8910,26
1,3810,09
1,5110,15
2,0810.10
0,4510,09
0,9910,20
1,5710,29
1,9710,32
2.5110,32
2,4910,46
Void Vol
(%)
28,7+0,8
30,5+1,4
30.911,2
32.0+1.6
33.1+1,1
35,111.8
29,510,6
30,611,1
33,110,8
35,1+0,5
37,511,5
38,411,1
28,410,4
29,110,7
31,5+0,7
32,511,1
35,0+0,8
35.510.8
29,210,4
30.510,3
32,6+0.9
33,411,2
34611,0
34,811,0
29.1+1.0
30,210,7
32,010,7
33,2+1,1
34,111.0
34.7+0,4
29,7+3,3
30,410,8
30,010,8
32,9+0,8
34,410,9
34,7+0,3
Water Abs,(%)
15,410,5
17,811,1
18,911,1
20,511,4
22.2+0,8
24,711,6
16,010,4
17,710,9
20.610,8
23,610.6
26,711,4
28,916,6
15,410,4
16,610,7
19,411,0
20,611,0
23.211.0
24,010,8
15,6+0,3
17,510,3
19,810,8
21.610,8
22,511,1
22,910,7
15,910,7
17,510,7
19,510,7
20,811,1
22.010.7
22,910.4
16,112,7
17.1+0,7
19,110,7
20,110,8
22.111.0
22,310,4
Density (g/cm')
1.86+0,02
1,7210,03
1,64+0,03
1,56+0.04
1.49+0,02
1,4210,02
1,84+0,02
1,73+0,03
1,6110.03
1,49+0,02
1,40+0,02
1,3410,07
1,84+0,03
1,76+0,02
1,63+0,05
1.58+0,03
1,5110,03
1,48+0,02
1,8610,02
1,75+0,02
,65+0.03
1,5510,05
1,54+0,03
1,52+0.02
1.8410,03
1.73+0,03
1,64+0,03
1,59+0,04
1.55+0.03
1,51+0,01
1,86+0,09
1,78+0,02
1,68+0,08
1,63+0,03
1,5610,03
1,55+0,01
*A11
composite were fabricated u sing ordinary portiand cement, air-cured for 28 days, tested at 50+5 per cent RH and 23±2°C,
* 3 standard d eviation, sample siz e n=9.
Of
the
six different pulp reinforced composites studied, the sample pulp which contamed
pulp with a Kappa number about 27 had highest strength value. This could be due to the
fact that Kraft 27 pulp contained the strongest fibre as measured by zero-span tensile index
(see Table 8.2). If the standard deviations are considered, one would not expect a
significant strength variation among
iie
pulps at the same fibre content. The small amount
146
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 164/220
of variation would be caused more by different fibre length rather than by different Hgnin
content. The influence of Hgnm content will be seen more clearly when discussing
composite fracture toughness property.
The fracture toughness values of the composites mcreased with mcreasing fibre content in
all cases. How ever, the rate of increase and the toughness values obtained varied greatiy
between pulps as shown in Figure 8.2. Aga in Kraft pulp at Kappa num ber 27 had the
highest toughn ess value and a high rate of increase. This could be due to the pulp having
the strongest and longest fibres in addition to good flexibility for mixing with the cement
matrix. TM P and CTM P pulp had similar high values for lignin content, low strength
index and high fines content, one w ould not expect composites reinforced with these pulps
to have good fracture toughness. The slightly longer CTM P pulp composite had better
fracture toughness than that of TM P pulp. The unsatisfactory fracture toughness value of
Kraft 17 W FRC m ight be attributed to the smaller and weaker reinforcing fibres compared
to those in Kraft 27 . Air-cured Kraft 27 WFRC had better toughness at high fibre contents
(say > 8%), although not much difference was observed between Kraft 44 and Kraft 27
WFRC at low fibre contents.
Bleached pulp composites even though they contain relatively short and weak fibres had
very competitive fracture toug hness w ith that of Kraft 44 and 27 com posites. This could
be due to bleached pulp fibre being more flexible and forming strong bonds to the cement
matrix. The high density values of WFRC reinforced witii bleached
pulp
supports this
statement. Fracture energy is the combina tion of the fibre-matrix bond breaking, fibre
fracture and fibre pull out. If the fibre is strong enough to w ithstand a pulling force, then a
well bonded com posite should have good fracture toughn ess property. It was discussed in
147
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 165/220
Chapter 7, that fibre strength had great impact on composite toughness and that the
toughness decreases rapidly with reduction in fibre strength. In that case, the fibre was not
strong enough to withstand pulling force and the fibre failure was the controlling factor
during specimen fracture. The properties
of
air-cured WFRC reinforced with bleached
pulp
suggested that the oxygen bleaching
to
Hgnin con tent down to about 0.7%
(in
addition to wet zero-span tensile index about 100 Nm/g) was still appropriate for air-cured
products.
1.8
1 6
1 4
1.2
0.8
^ ^
X
;•;
o
•
-
+
TMP ---- -^^^^ ~ ~ ^ ^ ^ S ^ ^
CTMP ^ ^ ~ ~ ~ ~ ' ~ « ^ - - _
"""**—
K44
K27
K17
Bleached
6
8
Rbre Content
(
by mass)
10
12 14
Fig. 8.3. Influence of fibre ignin content on air-cured WFRC density at different fibre content
The variation of composite density, water absorption and void volume was expected from
the flexibtiity natural of tiie pulps. The stiffness or flexibihty of the fibre mcreases with
the removal
of
the Hgnin, with flexibility gradually improving from TMP, CTMP, high
yield Kraft pulp, low yield Kraft pulp tiirough
to
bleached pulp . Fibres with
a
high degree
of flexibility are easy to form in the composite and provide good interfacial bonds with the
cement matiix . Such composites have high density and low water absorption and void
volume values
as
seen in Table 8.3 and Figure 8.3.
148
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 166/220
8 2 3 Influence of lignin content on autoclaved composites
Table 8.4 and Figure 8.4, 8.5, 8.6 shows the vanation of autoclaved WFRC mechanical
and physical properties influenced by fibre Hgnm content.
It can be seen from Table 8.4 and Figu re 8.4 that the flexural strengtii of autoclaved
composites reinforced with low yield Kraft pulp and bleached pulp mcreased with fibre
content increasing w hich was in agreement with that of the air-cured products. How ever,
the strength of W FRC reinforced with high yield TM P pulp, at all fibre contents showed
littie improvem ent. CTM P pulp composite at above 8% fibre by mass content showed
some small strength development.
30
25
20
5
i 15
m
I 10
0
6 8
Fibre Content ( by moss)
X
-X
o
-
+
-
-
TMP
CTMP
K44
K27
K 7
• ' C ^
Bleached
r '
r
z '
X
•
X
1
o
_-2—-—
X
X
10
12
14
Fig. 8.4. Influence of fibre lignin content on autoclaved W FRC flexural strength at different fibre content
149
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 167/220
Table 8.4 hifluence of fibre Hgnm content on autoclaved WFRC
Fibre (w%)
TMP, 25,52%
2
4
6
8
10
12
CTMP, 26,03%
2
4
6
8
10
12
K44, 6,47%
2
4
6
8
10
12
K27, 4,04%
2
4
6
8
10
12
K17, 2,55%
2
4
6
8
10
12
Bleached, 0,69%
2
4
6
8
10
12
MOR (MPa)
12,511.8
11,711.9
11.2+2,1
13,0+1,6
13,0+2,5
11,812.3
12,0+1,4
11,210,8
13,210,7
13,912,3
16,912.8
16,212,3
13,710.8
16,210.9
18,711,7
18,611,6
17,811,2
19,212,0
12,110,7
17,512,0
18,011,4
19,4+1,9
20,111,4
19,010,9
15,311,1
16,312,8
20,511,0
20,711.5
20,211,1
20,911,2
14,110,5
17,311,9
19,812,2
21,712,8
21,212,4
20,411,1
Frac.
Tough(]cJ/m2)
0,0710.01
0,1710,01
0,3310,05
0,4610,05
0,7310,16
0,7710,16
0,1310,04
0.4610.07
0,6610,09
0,7910,18
0,8910,20
1,06+0,18
0,13+0,02
0.45+0.10
1,0910,29
2,1810,20
2,6210,21
2.6510,67
0.0710,02
0.3810,05
0,6410,09
2,3610,41
2.8910,55
2.8210,12
0.1510,02
0.3210,06
0.6810,03
0.8810.02
1.3810.02
1.8410,01
0.1210,07
0,4410,06
0.5510.09
0.8110,27
1.7110.47
2.5310.27
Void Vol (%)
36,9+0,9
39,111,0
41,611,4
42,111.1
46,311,2
48,111.3
39,111,0
41,a+0,6
42,910,6
44,910,6
46,510,7
47,8+0,4
38,110,8
40.010,4
40,611,2
41,711,2
44,111,1
44,911,4
39,311,3
41,510,7
42,710.8
43,210,5
44,411,1
44,310,6
39,211.2
41,211,2
41,610,8
41.811.7
42,911.3
43,412.1
39,1+0,6
41,511.1
41,011.0
41,311.9
44,711.2
44,4+0.6
Water Abs,(%)
23,310,8
26,611,2
30,4+1,9
31,911,4
39,111.9
42,912,2
25,011,2
28,710,8
32,210,9
35,511,0
39,4+0,4
41,611,1
23,810,7
26,610,6
28,111,4
30,411.4
33,1+1.3
34,811.9
25,211.3
28,010.8
30,511.0
32.010,9
34,311.4
34,910.9
24.8+1.0
27,711.3
29,011.0
30,211.8
32.4+1.6
34.511,9
24,710.7
27.711.3
28.211.1
29.412.0
34.111,7
34.010.9
Density (g/cm )
1,59+0,02
1,4710,03
1,3710.05
1,3210,03
1.1910,03
1,12+0,03
1,56+0,03
1,4310,02
1.3310,02
1.2710,02
1,1810,06
1,1410,02
1,6010,02
1.5010.02
1,4410,03
1,3810,03
1,3410,02
1,2910,03
1,5610,03
1,4810,02
1,4010,02
1,3510,02
1,3010,02
1,2710,02
1,5810,02
1,4910,03
1,4410,02
1,3910,03
1,3210,03
1,2610.04
1,5810,02
1,50+0,03
1.46+0.02
1,4110.03
1.3110,03
1,3010,01
*The composites were fabricated using ordinary Portland cement and silica at the ratio of 1:1, autoclaved at 1,25MPa
steam pressure for 7,5h, tested at 50±5 per cent RH and 22±2° C, 3 standard deviation, sample size n=9.
It was discussed in section 3.3 and 3.4 that natural plant fibre (wood) consists of
considerable amount of Hgnin and this Hgnin can be dissolved by some chemical solutions
and the high temperature can accelerate the reaction
rate.
This is the principle of chemical
pulping. Composites subjected to autoclaving could easily dissolve Hgnin due to the high
150
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 168/220
curmg temperature (above
175°C)
and severe alkahne cement environm ent. This
dissolved Hgnin would eitiier poison the matrix curing or deposit on the surface of the
fibre causing poo r interface bon ding, or the combination of these two. The mechanism of
dissolving Hgnin is still not fully understood.
3.5
1
o
c
2.5
K 1.5 h
9
I
I 1
0,5
0
X TiMP
X
CTMP
O
K44
•
K27
-
K17
+
BIssched
-
.f^
1
^^
1 1 , -
^^^
_^
•
1
X
6 8
Fibre Conte nt ( by mass)
10 12
4
Fig. 8.5 Influence of
fibre
ignin content on autoclaved WFRC fracture toughness at different fibre content
Autoclaved composites reinforced with fibres containing low lignin appear to be more
stable than those with higher lignin content. Thus autoclved WFRC reinforced with
bleached pulp and Kraft 17 showed h igh flexural stiength as seen in Table 8.4 and M ai's
work (1983). Autoclaved TM P fibre reinforced com posites had the lowest flexural
strength values due to the effect of Hgnin dissolving / depositing. The sam e observation
was reported by Coutts (1986), he suggested that high yield TMP and CTMP pulps were
unsatisfactory for use as a altemative fibre reinforcement to P radiata Kraft pulp in
autoclaved cement mortars, but acceptable for use in air-cured products.
151
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 169/220
The effect of lignin content on the property of fracture toughness could be understood in
terms of fibre strength. Fibre
sttength
plays a dominant
role
to the composite fractiire
toughness as discussed in Chapter 7. Figure 8.2 and 8.5 shows composites reinforced with
strong fibre (Kraft 27) had the highest toughness value.
This remark is strengthened by tiie results of the fibre streng th study in Chapter 7. The
fibres in the presen t study had a variation in strength properties (Table 8.2). In addition to
dissolving Hgnin, autoclaving is akin to a severe alkaline pulp ing and can further
reduces strength values due to degradation of fibre cellulose. Although the degree of such
strength loss is uncertain, the fracture tou ghness loss of autoclaved composites reinforced
with bleached pulp was significant com pared with air-cured produc ts. A few w orkers
(Lhoneux, 1991; Stevens, 1992) have studied fibre strength loss during autoclaving,
mainly using lime solution to simulate the matrix environm ent. This study attempts to
investigate the real pulp ing conditions present in the autoclave by means of a
reconstituted handsheet techn ique. Reinforcing fibre was first formed into handsheets,
then inset into cem ent blocks subjected to autoclave; after autoclaving, the matrix was
broken apart, the handshee ts were carefully removed out and disintegrated into pulp; then
characterised for change of pulp strength and other properties. In our preHminary study it
was observed that tiie fibre strength and lignin con tent were reduced during autoclaving,
however extensive work is required and results wiU not be reported in this thesis.
The variation in autoclaved composites physical properties were similar to those of air-
cured cement products. The fibre stiffness or flexibility, wh ich is influenced by the fibre
lignin content, became the key factor in regarding materials density, water absorption and
void volume as listed in Table 8.4 and shown in Figure 8.6.
152
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 170/220
1.8 I-
1 6
I 1.2
a
1
0.8
0.6
6 8
Fibre Con tent ( by mesa)
10
12
14
Fig. 8.6. Infiuence of fibre lignin content on autoclaved WFRC density at different fibre content
8.3 Conclusions
The results showed that both TMP and CT M P pulps did not have satisfactory mechanical
properties when used to reinforce autoclaved cement composites. Air-cured products have
better mechanical properties and could be possible used in a number of apphcations such
as renders, moulded articles or sheet products.
The Hgnin compon ent of the pulp fibre had considerable influence on tiie composites
mechanical and physical performances. Such influences could be understood m terms of
fibre strengtii, fibre stiffness, fibre-matrix interface bo nding , fibre number and lignin
dissolving mechanics during the autoclavuig process.
153
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 171/220
Experimental results also showed that composites reinforced with Kraft pulp with Kappa
number around 27 had the best mechanical and physical properties in both air-cured and
autoclaved products.
154
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 172/220
Chapter Nine:
Conclusions and Further Work
9 1 Conclusions
The following conclusions can be drawn from this work:
1. Chem ically pulped bamboo fibre is a satisfactory fibre for incorporation into a cement
matrix. M echanical and physical properties of both air-cured and autoclaved bamboo fibre
cement com posites were studied. Bam boo fibre cement products have reasonable flexural
strength and competitive physical properties to their wood fibre cement counterparts.
However, the fracture toughness values are low due to short fibre length and high fines
content of the bamb oo pulp. Improved prope rties of composites reinforced with screened
long bamboo fibre confirm this
belief.
There is little difference between the properties of composites reinforced w ith beaten and
unbeaten bamboo pu lp. This could be due to the average short fibre length or the structure
of bamboo fibre when compared with wood fibres.
2. Properties of bamboo fibre cement composites (and the other natural fibre products) can
be improved by blending long fibres (such as softwood fibre) w ith the bamboo
reinforcement to increase the average fibre length. Such hybrid fibres improve the
composite strength and more importantly the fracture toughness value. The extent of such
improvement is related to the increasing proportion of long fibres.
155
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 173/220
3.
The influence of fibre length on composite performance has been extensively studied.
Fibre length plays a significant role in the behaviour of fibre cement composites. If the
fibre is long, a greater amount of fracture en ergy is needed to
puU
the fibre through the
matrix and the com posite can be tougher and stronger. This work was carried out by
fractionating a single pulp mto a range of lengths. The
resuHs
showed that when short
fragments of fibre (weighted length 0.30 mm) were used as reinforcement and compared
with longer fibres (weighted length 1.59 m m ), a drastic reduction in composite fracture
toughness and strength was observed. For example, at 8% by mass the short fibre samples
showed a seven fold decrease in fracture toughness and possessed only half the flexural
strength of samples containing the same amount of the longer fibre.
4. Research w ith holocellulose fibre pu lp, chemically treated to vary the tensile strength of
the individual fibres, indicated that fibre strength was particularly important to composite
toughness but not to composite strength. In both cases of air-cured and autoclaved fibre
composites, when fibre zero-span tensile index (wet) and pulp viscosity values were
reduced to about 40% of fuH strength, the comp osites fracture toughness va lue also were
observed to
faU
to about 40% of original value. Surprisingly, the flexural strength values
were virtually unchanged. The reason behind this behaviour is not clear. Exam ination of
SEM micrographs, of the composite fracture surfaces, showed that the samples containing
the weaker fibre had produced the expected higher population of broken fibres than
samples containing the stronger fibres.
5. Air-cured and autoclaved composites remforced with a wide range of Hgnin content
pulps were evaluated. The Hgnin comp onen t of the pulp fibre had a great influence on the
com posites mechanical and physical perform ances. Such influences could be understood
156
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 174/220
in terms of fibre strength / fibre stiffness (cellulose content), fibre-matrix mterface
bonding, fibre number and lignin dissolving mechanics during the autoclaving process.
Both TMP and CTMP pulps did not have satisfactory mechanical properties when used to
reinforce autoclaved cement composites. Bleached pulp reinforced composites had high
strength but low toughness values (compare to Kappa num ber 27 pulp ). The composites
reinforced with Kappa number of about 27 kraft pulp had the best mechanical and physical
properties in both air-cured and autoclaved products.
9.2 Recommendat ions for further work
9.2.1. Pulp supply
The reinforcing potential of kraft fibres depends m ainly on the properties of the raw
material used. Althou gh fibre properties can be affected by the cooking conditions - alkali
charge affecting wet fibre flexibility and sulphidity affecting intrmsic fibre strength - these
factors do not compensate for tiie variations caused by the raw material. It is therefore
essential for both the final product quality and production costs that the interactions
between raw material and end-products properties are
weU
understood and the wood fibre
property variations can be measured.
The basic natural (eg. wood) fibre p roperties are wall thickness, fibre width, fibre length,
weak poin ts in the fibre
waH, S fibril angle, tiie index of crystalHsation and chemical
composition. These properties vary between different wood species; within one annual
ring (springwood and summerwood); between different parts of the trunk and are due to
the different growing conditions.
157
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 175/220
Modem forestiy has accelerated forest growth, and the trend is to harvest younger trees.
The raw material is also used more efficiently (sawmiU chips, whole tree chips and wood
residue), and the am ount of imported chips from plantations seem s to be increasing. This
means that quality variations in raw material are also increasing significantiy. If this is not
taken into account in pulping and stock preparation, the reinforcing potential of the raw
material is not utilised in fuU, and at the same time final product quality variations
increase.
Tasman Pulp and Paper Co. Ltd., New Zealand has made progress in matching the highly
variable wood supply to various end-products, i.e. papermaking and fibre-cement, by
means of segregation of tiieir fibre supply into density ranges. Howev er, there is still a
long way to go to understand the true relationship between fibre quality and end-products
properties and to modify the fibre quality to match the products requirement (Williams,
1994).
9.2.2. Fibre length population distribution and cross-dimensions coarseness)
Among the wood fibre basic properties, fibre length and fibre cross-dimensions, i.e. wall
thickness and fibre width, might be the most important factors regarding the preparation of
paper and fibre com posite produ cts. The influence of fibre length has been studied in this
thesis, however, the influence of fibre length population distribution would be more
meaningful to fibre-cement co mp osites. This is because of the nature of non-uniformity of
fibre length, the nature of stock preparation and pulp blending.
The fibre cross-dimensions, i.e. waH thickness and fibre width, has often been
characterised using the coarseness value. Because of the close relationship between the
158
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 176/220
cross-dimensional properties of softwood fibres and fibre length, the variation in paper
(and fibre-cement) properties caused by the ceU wall thickness and fibre width have many
times been erroneously explained in terms of fibre length. The different cross-dimensiona l
properties of fibres, for example thick-walled, narrow, stiff and strong fibres and thin-wall,
wide, flexible with a large lumen and
exceUent
bonding abihty fibres, has great impact on
their end-p roduc ts properties. The influence of fibre cross-dimensional characteristics on
paper properties has been studied by a number of researchers (Clark,
1962;
Seth,
1987;
Kibblewhite, 1989), some people even suggested that in practice the cross-dimensions
(coarseness value) was the most identical characteristic to predict the papermaking
potential of different softwood fibres (Paavilainen, 1993).
Very limited work has been done regarding the effect of fibre cross-dim ensions to the
fibre-cement prod ucts quality. Vinson and Dan iel (1990) demonstrated that high density
summerwood fibres (coarser) were better suited for fibre cement reinforcement than low
density fibres. Coutts (1987) in studying natural fibre reinforced cement com posites
suggested the importance of fibre dimensions in respect to its length and diameter aspect
ratio. How ever, mo re extensive work is required before a clear picture can emerge.
9.2.3 Conformability - flexibility and collapsibility
Fibre conformabihty, namely fibre flexibility and collapsibihty, characterises the abihty of
wet fibres to deform (p lastic and elastic deformation). Both fibre flexibility and fibre
collapsibility are dependent on cross-dimensional fibre properties and on the elasticity of
the cell wa ll. At the same time the fine stiuctu re, chemical com position and water content
of the cell wall as well as the drying history and the chemical and mechanical treatments to
which the fibres are subjected
aU
play a role in how the fibre will respond.
159
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 177/220
In the case of chem ical pulp fibres, it is widely agreed tha t collapsibility and flexibility are
important both for paper and composites properties . Furthermore the conformabihty of
the fibre also affects stock preparation and machine run-ability.
Although the imp ortance is widely recog nised, know ledge of tiie factors affecting
conformability is limited and not systematic. Also, experimental verification of the
relationship between conformability and end products properties is difficult to find in the
scientific literature (Paavilainen, 1993).
9.2.4 The impa ct of pulp med ium consistency treatm ent on fibre-cement products
The processing of pulps at medium consistency (MC) has become common in the pulp and
paper indus tiy. The M C technology , which employs fibre mass concentrations typically in
the range 8-14%, allows considerable reduction in the volume of pulp suspension handled
during processing op erations. This has led to reduced equipmen t size, and saving in
capital and operating costs.
The conditions required to fluidise the suspensions lead to beating, as well as the
introduction of curl and microcompression in the fibres (Seth,
1991).
The MC treatment
will result in a higher sheet stietch and lower elastic modu lus of the sheet. Sheet porosity
and tear wiU also be enhanced by the increased curl and microcompression (Page, 1985;
Seth, 1993). Althoug h the influence of curl on the fibre cemen t composites has been
studied by Michell (1990), the implications of MC processing nevertheless needs further
investigation with both bench-scale and commercial equipment.
160
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 178/220
9.2.5 Theoretical modelling
There is Hmited work available on model systems in the Hterature, dealing with fibre
reinforced cement composites. A consideration should be given to the developm ent of
models to predict composite properties based on an understanding of fibre and matrix and
their inter-relationship. M odelling allows the engineer to take into consideration sca ling
and size effects. It also has the advan tage of being able to indica te the direction for future
work in achieving optimum strength and toughness performance for a given system.
161
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 179/220
Appendix A:
Fabrication and Characterisation Methods
A.l Pulp Fibre Preparation
A.1.1 Chem ical pulping Kraft pulping)
The Kraft process is the predominant pulping process in industry today; it is tolerant to
variations in wood chip dimensions and wood quality and, because of its high strength,
Kraft pulp can be used in a wide range of end products.
The Kraft process involves heating the wood at 165-175°C with a solution of sodium
hydroxide (NaOH) and sodium sulphide (Na2S) for 0.5-3 hours. Time to cooking
temperature is in the range 1-2 ho urs. Pulp yield 45 -55 % . On an industrial scale, the
spent cooking liquor (called black Hquor) is
concenttated
and then burnt in a fumace,
allowing the chemicals to be recovered and at the same time providing energy for the pulp
min.
In the laboratory assessment of pulpwoods, pulping conditions (temperature, time) are
chosen to resemble those of industry. Wood sam ple size are usually constant and
specified in terms of oven dry mass. As the volume of wood contained in each vessel
fluctuates with the density of the wood sample, sample sizes must be chosen so as to
remain within the capacity of the pulping ves sels. In the air-batii used in this work six 3-
Htre pulping vesse ls a re used (Fig. A. 1). Typical conditions for a cook would be two
hours at 170°C, 25%o sulphidity and 4:1 liquor to wood ratio.
The alkali charge is varied to achieve the required degree of delignification,
usuaUy
described m terms of Kappa number ( a measure of the residual lignin content, see section
A.2.2.1). Unbleached packaging grade pulps are usually delignified to Kappa numbers
around 40, bleachab le hardwood pulps are lower, about Kappa 20. Pulps for
reinforcement in the cement composites their Kappa number are in the range of 30 to 20.
162
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 180/220
PULPIH
VESIELS
LO KIM
FLIN
TMtHHOWr lr
fO K T
LO K IX
• IN
Fig. A.l. Schematic illustration of Air-bath and 3-litre
pulping
vessels.
The active chemicals in the Kraft pulping liquor are sodium hydroxide (NaOH) and
sodium sulphide (Na^S). The alkali charge is usually expressed in terms of the equivalent
quantity of sodium oxide (NaoO), although that particular chemical entity is never actually
encountered. Three param eters are used to define the chemical make up of a Kraft pulping
liquor: active alkali, effective alkali and sulphidity.
1. The active alkali (abbreviated AA) is NaOH +
Na2S,
expressed as Na20, and usually as
a percentage relative to the weight of oven-dry (o.d.) wood chips to be cooked.
2.
The effective alkali (EA) is defined as NaO H -i- l/2Na2S, expressed as
Na20 .
3.
The sulphidity is the percentage ratio of Na2S to active alkali, expressed as Na20.
For most new wood samples the
alkaH
charge required to attain the target Kappa number
will not be know n. It is usually necessary to find the proper alkah charge by trial. As the
change in Kappa number with alkali charge is frequently non-linear at low Kappa
numbers, experience with similar wood samples is often the only guide. The normal alkali
requirement for softwood pulping is about 12 to 14% effective alkali on o.d. wood [8 to
10%o
for hardwoods (Smook, 1982)].
163
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 181/220
The
aUcali
charge is
usuaUy
made up from stock solutions of sodium hydroxide and
sodium sulphide. These stock solutions have to be standardised before use because tiie
chemical concentrations change as the chemicals react with carbon dioxide in the
atmosphere.
Ano ther essential piece of information is tiie mo isture content of the wood sam ple. Not
only does the moisture content affect the weight of wood required hi each pulping vessel,
bu t also the Hquor to wood ra tio. Details of the calculations required to determine the
amount of wood, chemicals and water required for a Kraft cook are given in Appendix B.
The 3-litre air bath can be preheated to a temp erature well in excess of the final cooking
temperature before the pulping vesse ls are loaded. The air bath can also be heated after
loading the pulping vessels. It usually takes 1 .5 -2 hours to reach the cooking tem perature
in this case. Although the air bath can be thermostatically controlled w hen finally at the
cooking temperature, the rise-to-temperature portion of the cycle, especially from about
160°C on, is controlled m anua lly, and some degree of judgement is required to effect a
smooth approach to the cooking temperature without significant overshoot. It is suggested
that the maximum temperature up to
180°C does not significantly affect the cooking
result. From 180 to 190°C , there appears to be a small reduction in yield; above 190°C,
the yield and strength loss may be substantial due to attack on ceUulose (Smook, 1982).
Temperature reading are taken at intervals throughout the cook to allow the calculation of
the
H-factor. H-factor,
first developed by Vroom in 1957, is a means of representmg the
times and temperatures of any cook ing cycle as a single num erical value. Its values lies in
aHowing the comparison of pulping procedures which incorporate differing temperature /
time profiles.
At the completion of the cooking cycle, the pulping vessels are promptly removed from
the air bath and coo led in a bath of cold water to halt
furtiier
deHgnification.
164
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 182/220
After cooling, the spen t cooking liquor (black Hquor) is drained off. The softened chips
are given a brief wash in cold water to reduce foaming, then the chips are disintegrated.
The method used for disintegration often depends on the mdustrial process being
modelled. At CSIRO, the softened wood chips from the 3-Htre air bath are disintegrated in
a mixer at 2850 revolutions per minute for 10 minutes.
FoUowing disintegration, the pulp is washed with cold water in vacuum / de-water funnels
to remov e residual b lack liquor. This is a critical step because residual black liquor will
affect the Kapp a num ber and pH of the pulp. Th e washed pulp is then screened to remove
uncooked fibre bund les. At CSIRO , a Packer screen w ith 0.2 mm wide slots is used for
this purpose. The screened pulp is then dewatered, usually w ith a pres s, to about
15
consistency, follow with crumbing. The crumbing, which can be done in a large Hobart
mixer, assists in distributing the moisture in the pulp evenly. This allows an accurate
estimate of the mo isture content of the pulp from small sam ples. The mo isture content of
the pulp can be measu red at this stage. The crum bed pulp is bagged in a sealed plastic bag
and stored in a refrigerator to maintain the moistu re co ntent.
A.1.2 Mechan ical pulping TM P, CTM P)
A.
1.2.1
Asplund
defibrator
Mechanical pulping in its various forms has been
clauned
as the pulping method of tiie
future. The principle of mechanical pulping is mechanically sepa rating wood into its
constituent fibres. Chips may first be steamed or pretreated with 6-10% sodium hydrox ide
(Na- +)
or calcium hydroxide
(Ca+- )
to soften lignin and to make the separation of the fibre
easier.
In this experimental study, thermom echanical pulp (TM P) and chemithermomechanical
pulp (CTMP) are prepared in Asplund Type D Laboratory Defibrator, which is equipped
with facihties for pre-steaming and subsequently disintegrating wood chips under gauge
pressures up to 12 atmospheres (190°C) as described by Higgins (19 77). The main
variables in TM P (CTMP) are the raw material (species, basic density, chip moisture
165
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 183/220
content and chip geometry), steam pressure (and corresponding temperature), pre-heating
time and defibration time.
The chip charge can be as high as 400g (o.d.) wood, but in practice (e.g. hardwoods) it was
sometimes necessary to run with a charge as
low
as
lOOg
to avoid overload on the 7.5 kw
motor.
P.radiata
low temperature range pulps (125-135°C LT-TMP) was reported to yield
well fibrillated fibres with good paper-making properties; high temperature pulps
(150-
170°C HT-TMP) to yield smooth, lignin encased, unfibritiated fibres (Higgins, 1978).
To operate the Asplund defibrator, wood chips are placed in the four compartments of the
inner vessel in approximately equa l amo unts, tighten the lid, check the temperature. After
presteaming the chips for 1-2 minutes at low temperature range (120-125°C), start
defibrator for 2-3 minutes (1440 rpm) to break chips down into fibre bundles. These fibre
bundles are fed through Bauer refiner to form individual fibres (see section A.
1.2.2).
For chemithermomechanical pulping (CTMP), chips are soaked in 10% caustic soda
(NaOH) over
18
hours at am bient tempe rature and then processed as in the case of LT-
TMP pulps.
A. 1.2.2 Bauer refiner
Refining reduces a fibre bun dle to individual fibres. The key part of a
refuier
is the
refining plates. The refining plates tiansmit the mechanical energy into the wood fibres,
so the primary function of plates is to keep the pulp between the plates.
Refining of Asplund defibrated pulp is done in the Bauer refiner fitted with 203 mm (8
inch) diameter plates (rotor and stator). Althou gh there are two types of refining plates
(open periphery and closed periphery plates) (Fig. A.2), Asplund defibrated TMP (CTMP)
fibre bundles can be refined to individual fibres by use of closed periphery plates.
166
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 184/220
jS^^^ ^^^
j ^ ^ ^ ^ g ^ ^ ^ f
?? r> f o p-
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ g ^ ^ ^ ^
lii^'>««^^^^>^^^^^^^B£^^'^' '^^^'^^iJ^^^^^^^Hl[^^'^^^^^^^^^^^H
^g5^^Sf ^*tf^*^^a
^Ki
H^^c c **
ROTOR ^ ^ ^ ^ ^ ^
Fig.
A.2. Open
periphery (C) and
closed periphery (B)
refining plates.
Refining is achieved by successive passes of the pulps at various plate clearance until the
required freeness level is reached. Refining Asplund defibrated
P.radiata
LT-TMP pulps
in Bauer refiner fitted with closed periphery plates (rotor: 8117-4122p, stator:
8117-
4121p) can be done using the following passes. One pass is made at a plate clearance of 2
mm (0.08 inches), one at 1.25 mm (0.05 inches), one at 0.625 mm (0.025 inches) and two
at 0.125 mm (0.005 inches).
Pulp mo isture consistency is a major factor to refining efficiency. W ater should be kept to
a minimum, if too much water is used (consistency < 8%o), no work is done on tiie fibres
and therefore no refining takes place. Care should be always taken during reducing the
plates clearance not to clash the plates.
A 1 3 Bleaching pu lps (Oxyg en delignification)
The main objective of bleaching in papermaking is to convert a dark coloured pulp into
one which is much tighter in colour. Bleached pulp imp roves fibre cement com posite
flexural strength due to fibres are more flexible and better fibre-matiix bonds (Mai, 1983).
Chemical pulps contain residual Hgnin. This lignin has been extensively modified by tiie
severe conditions used to make the pulps and can be quite dark in colour. It is extremely
difficult to change this Hgnin into a colouriess form with bleaching chem icals. So tiie way
to bleach a chem ical pulp is to completely remove the residual Hgnin. This is done in
multistage processes using chlorine compounds or oxygen for lignin degradation, and
167
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 185/220
alkali for extraction of the degraded Hgnin. Often mixtures of bleaching chemicals or
sequential addition of different bleaching chemicals is used ui the same bleaching stage.
To simplify the description of these sequences, each chemical can be designated with an
appropriate letter. Tab le A l Hsts com mon bleachmg chem icals, their usual designation
and their bleaching action.
Table Al Common bleaching chemicals
Chemica l
Chlorine,
Ch
Hypochorite,
(NaClO
or
CaClO)
Chlorine dioxide, ClOo
Alkali, NaOH
Oxygen,
O2
Hydrogen peroxide H9O9
Designation
C
H
D
E
0
p
Bleaching action
Lignin degradation
Lignin degradation
Lignin degradation
Extraction of degraded lignin
Lignin degradation or improved delignification in E-stages
Improved delignification in E-stages
The choice and the conditions of use of the bleaching chemicals are limited because
carbohydrate degradation must be avoided. Otherw ise, pulp yield would be reduced and
strength prope rties impa ired. There are few co mm on laboratory bleaching sequences such
as CEHD and 0D,(E0)D2.
The effect of fibre Hgnin content level on WFR C p roperties is studied. Low Hgnin con tent
in the fibres was achieved under oxygen delignification conditions. The pulp responds
weH to oxygen bleaching and about half of the Hgnin can be removed easily. The general
rule for oxygen bleaching, particulariy when applied to softwood Kraft pulps, is that about
40 per cent of the Hgnin can be removed
before
the strength properties of the pulps are
affected. The viscosity of the oxygen bleached pulps was measured as this property can be
indicative of fibre damage if the value is below a threshold level. A viscosity of about 22
m.Pa.s is regarded as the lower limit for an oxygen-bleached northern hemisphere
softwood Kraft pulp (Teder, 1991).
Oxygen deHgnification of the Kraft pulps was carried out with the 3L pulping vessels
which were fitted with lids incorporating valv es to introduce oxygen into the vessels. Pulp
samples (lOOg o.d.) were mixed with magnesium carbonate (1% pulp basis), sodium
168
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 186/220
hydroxide (in the range 0.7-2.5 per cent, pulp basis) and water to give a pulp consistency
of 10 per cent. The mixture s were placed in the pulping vessels which were p ressurized
with oxygen (780 kPa) and heated at 115°C for 30 min (time to temperature was 75 min).
A 4 Holocellulose pulp
HoloceHulose is defined as lignin free, pure cellulose fibres. Holoce llulose is used to
study influence of fibre streng th on W FR C prope rties. This Hgnm free fibre allows further
Kraft cooking to various fibre strength without the complication of different lignin content
products, which might effect the fibre strength determinations.
Sodium chlorite solution at room tem perature was used for the delignification. The
solution consisted of 60 g sodium chlorite, 20 g anhydrous sodium acetate and 40 ml of
glacial acetic acid, made up to a litre witii purified w ater. The mo ist, unbleached Kraft
pulp at about 20 per cent consistency is mixed with sufficient of the chlorite solution to
give about 5 per cent consistency and allowed to react at room temperature with
occasional mixing and shaking for 24 hours. The pulp was then washed and the chlorite
treatment is repeated w ith fresh solution for another 12 hours. After chlorite treatment the
pulp is washed thoroughly with purified water (Stone, 1960).
A 1 5 Beating
The basic purpose of beatmg is to mechanically condition tiie fibres for papermaking and
manufacturing W FR C. Addition to tiiis, beatmg plays an important role in the Hatschek
process to retain cemen t and silica particles (Coutts, 1982a). A more general term for
mechanical working of pulp is refming . The term beating actually denotes a specific
type of refining, but is now com mo nly used to describe refming in the laboratory. Most
laboratory beating methods have a more selective action than mill refiners and produce
results tha t normaUy cannot be dupHcated in the miU. So it is necessary to optimize the
refining level with mill equipment and conditions.
169
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 187/220
A num ber of laboratory beating device s are in use around the world. The two devices
most commonly used are the VaUey beater and the PFI miU. The VaHey beater (Fig. A.3)
is essentiaUy a min iature version of the Hollander beater. Although this device has a long
tradition of use, it has some definite limitations and is gradually being displaced by the PFI
mill. The principa l disadvan tage is the difficulty in obtaining reproducibility with respect
to other Valley beaters and with respect to the same beater over long periods of time. The
problem relates to variable wear patterns on the metal cutting edges. How ever, a Valley
beater is used in our experiments to prepare pulps for composite fabrication because
sufficient amount of pulp can be treated in a single run. To operate the Valley beater, a
soaked 360g o.d. pulp with 23 litre water is beaten without load for 20 minutes then beaten
for further period of time with the bed-plate load of 5.5 kg until the desired freeness level
is obtained. After beatm g the pulp is subjected to dewatering, pressing and crum bing.
at«r
oll
clghti
Fig. A.3. The Valley Niagara beater
170
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 188/220
cdpbfe
Fig. A. 4. The PFI laboratory beater tackle
The PF I mill utilizes a grooved roll eccentric to a smooth trough , as illustrated in Fig. A.4.
Both the roll and bed-p late rotate at high speed but at different peripheral velocitie s; this
action induces friction, rubbing and crushing of the fibres to produce the beating effect.
Since there is no metal-to-metal contact and no edges to wear, the device does not require
calibration. It also has the advantage of requiring a relatively small amount of pulp to
carry out a complete evaluation.
A 1 6 Preparation of fibres from dry lap-pulp
In
miU
practise, fibres are usually prepared in tiie pulp
miU
and transported in a dry lap-
pulp form. In the fibre-cement plant, the dry lap-pulp is soaked in tiie water to release
hydrogen bonds then disintegrated into pulp. Fibres experienced dry-lap distingentation
process may lose some initial strengtii. However, it is mo re economic and convenient for
the fibre-cement plant which has no pulping
factiity.
In our experimental work, some fibres were prepared from commercial dry lap-pulps or
packaging pap er. These lap-pulps or paper were torn into relatively sm all pieces and
171
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 189/220
soaked in cold water over night. Then they were disintegrated in a mixer at 2850
revolutions per m inute for 10 min. Following disintegration, the pulp was dew atered,
crumbled and stored in the refrigerator until composite fabrication. The pulp moisture
content can be calculated at this stage.
A.2 Pulp Fibre Characterisation
A large number of testing methods are in common use to characterize pulps with respect to
quality, process abihty and suitability for various end uses. Many of these test procedures
are emp irical in nature and prov ide useful behaviour information. Other more
fundamental tests provide the means to predict behaviour, or to explain and rationalize
the emp irical test results. A summ ary of common test methods is given in Table A2 .
Some of these methods were carried out during work for this thesis, the others are
recommended for further study (see Chapter 10).
Table A2 Pulp test methods (Smook, 1989)
Fundam ental Properties Empirical Tests
* weighted average fibre length * Kappa number
* intiinsic fibre strength * CED viscosity
* fibre coarseness
*
colour and b rightness
* specific surface area * cleanline ss
* wet compactability * drainability
* pulp chemical compo sitions * beater evaluation
A.2.1 Lignin content
A.2.1.1 Kappanumber
The non-cellulosic components (especially Hgnin) react readily with acidic permanganate
solution (KM n0 4). This reaction provides the bases for the Kapp a number test. At a
controlled tem perature, an excess of acidic perm anganate is added to the pulp to be tested,
and allowed to react for a set time interval, after which the unreacted permanganate is
reduced by an excess of iodine. The Hberated iodine is then determined by reaction with
thiosulfate. The Kappa number of the pulp is equal to the num ber of ml of acidified 0.02
M potassium permanganate solution which would be consumed by one gram of moisture
172
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 190/220
free pulp in
10
min at 25°C. The results are corrected to 50% consum ption of tiie
permanganate added, which ensures a satisfactory relationship to the Hgnin content of the
pulp.
The test procedures to b e
foUowed
is described
ui
detail in the
TAPPI
T236cm-85. This
method may be used for aU types and grades of chemical and semi-chemical unbleached
and semibleached wood pu lps in yields under 60 per cent. How ever, it should be noted
that reproducibility is less for high yield pulps than for low yield pulps. For pulps such as
TMP and CTMP, Klason lignin method wiU be more suitable.
A.2.1.2
Klason lignin
Klason lignin is defined as those components of wood or pulps which are insoluble after
treatment with 72 per cent m / m sulphuric acid followed by boiling in 3 per cent sulphuric
acid. TM P and CTM P pulps are more suited to the Klason lignin method (APPITA
PI
Is-
78). In this standard, the lignin content should not be less than 1 per cent to provide a
sufficient amount of lignin, about 20 mg, for accurate w eighting. It is not applicable to
bleached pulps containing small amounts of lignin.
Most woods contain some lignin which is rended soluble by the above treatment and
which is not determined by this standard. In softwoods and sulphate pulps this soluble
Hgnin content is small, about 0.2 to 0.5 per cent, but in hardwoods it can amount to 5 per
cent. Thus hardw ood w hich has had any alkali treatmen t, may give a lower results than
would be obtained from the untreated wood.
A.2.2 Dra inability Freeness)
The resistance of fibres to the flow of water is an important property with respect to pulp
processing, paperm aking and fibre-cement comp osite ma terials fabrication. The classical
method of determining this property in North America and Australia is by means of the
Canadian Standard Freeness (CSF) test (Fig A5). The CSF is defined as tiie number of ml
173
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 191/220
of water collected from the side orifice of the standard tester when pulp drains through a
perforated plate at 0.30% consistency and 20°C.
Measurement of pulp drainage are know as freeness, slowness, wetness or drain time,
according to the instrument or method used. If
a
pulp drains rapidly, it is said to be free
If it drains slowly, it is said to be slow . Freeness and slowness scales have an inverse
relationship. The Schopper-Riegler Slowness test is the principal drainage measurement
used in Europe and Asia.
CHAMBED
lACKINCfLATC
SC ECN
PL»TE
OlfCR
BR CKET
^ : :
LOWER BR CKET
SPRE OtB
COME
fUNNEl 1 ^ g
SIDE
T J ^ W
ORiricE
~ "
— Si
\
fl
CUD
Fig. A.5. Canada Freeness tester
OLUG
BOTTOM
ORir ic E
Freeness measurements are widely used as an indication of quality for mechanical pulps
and as a measure of the degree of refining (beating) for chemical pulps. Studies have
shown tiiat th fines fraction (-200 mesh) is primarily responsible for changes in drainage.
The removal of the fines fraction from beaten pulps can restore the original drainabiHty,
while the pulp retains its beaten strength properties. These
findings
are sometimes used as
an argument against the use of drainage measurements as an index of pulp quality (Smook,
1982)
174
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 192/220
Although freeness measurements provide a basis for comparing similar pulps, the test does
not simulate what happens on the paper machine wire. For example, groundwood pulp
gives a lower freeness than highly beaten chemical pulp but show s faster drainage on the
paper machin e. Furthermore, the same freeness value does not indicate the same degree of
fibritiation. Bamboo pulp might have same freeness value as a softwood pulp, but the
degree of fibrillation for the bam boo pulp would be much less than that for wood pulp due
to the fact that the bamboo pulp has o r average short fibre length, massive pitch fines and
sensitive response to the beating force.
A.2.3 Fibre length
Fibre length is measured or indicated either by microscopic examination of a
representative number of fibres or by screen classification of a sample into different length
fractions. In the microsco pic me thod, a known w eight of fibres is projected onto a grid
pattern; all the fibres are measured and the average fibre length is calculated
mathematically.
In the classification me thod, a dilute dispersion of fibres is made to flow at high ve locity
parallel to screen slots, wh ile a much slower velocity passes through the slots. In this way,
the fibres are presented lengthw ise to a series of screen w ith successively smaller mesh
openings, and only the fibres short enough not to bridge the opening pass in to the next
chamber. The Bauer-M cNe tt Classifier is one of the traditional instruments (Fig. A.6).
In the last few years, a new optical device, the Kajanni FS-200 Fibre Length Analyzer has
become available for measuring fibre length. It is now widely used and is becoming
accepted as a standard laboratory fibre length measurement (TAPPI
T271pm-91). The FS-
200 measures fibre length by an optical technique using polarized light and is based on the
birefringence of the wood fibres (Fig. A.7). The machine em ploys a measu rement range
of 0 - 7.2 mm, divided into 144 classes, each of which represents a 0.05 mm interval in
length. W hen the pulp samp le (average number of fibres
15,000
to 30,000) passes through
175
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 193/220
the analyzer, the num ber of fibres m each classification is counted. This data is fed to a
microprocessor unit which routinely records, calculates and displays the fibre length
average in tiiree modes: arithmetic,
length-weighted
and
weight-weighted.
Water Inlet
Constant Water Level
Supply Tank
Drain Pluq
Water Overflow
Overflow Water
to Drain
Overflow Water
to Drain
•Terylene Cloth
Muslin)
Fig. A.6. The Bauer-McNett Classifier
Stirrer
f d
ptics
Capillary
\
I
• I] U t • I L H ^
I
\ i «w
0
Detector
Laser
eaker
Fig.
A.7.
Kajanni
FS-200
measurement principle
176
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 194/220
There is no singulariy accepted definition of what is the average fibre length of pulp in a
sam ple. The generally agreed approach is to the definition best suited to the nature of the
sample under consideration.
If the sam ple is made up of fibres which are of
fakly
uniform length, the numerical or
arithmetic, average fibre length L^ is most appHcable (1). How ever, this definite is not
applicable wh en, as is mo st frequently the case, the sample contains a high proportion of
short fibres or fines. The use of a measurem ent which is weighted according to the
weights of the fibres is then preferred. The length-weighted average fibre length Li. , is
defined for the case where the fibre coarseness, the weight per unit len gth ,/, is assumed to
be constant (e.g.
/j
= C ) (2); the
weight-weighted
average fibre length, L. ,-, is defined for
the case where the fibre coarseness is assumed to be proportional to the fibre length (e.g./;
=
C k
)
(3).
L ,
= Sl/N
=
I n
J./Snj
(1)
h^ = Iwjl;/Iwi = I(nil:C)Vl(niljC) = In il ^/ In jl (2)
L^, , = Iw JJ lW i = I(nM)li/I(nili^)=
lXCnd^^/l Cn^^)=
lAd^ lA^^ (3)
Where 1 represents th e average length of fibres in the i* fraction, n represents the number
of fibres in tiie i'' fraction and Wj represents the weight of fibres in tiie i fraction.
According to Clark (1985), the coarseness of natural fibres increases with fibre length and
he therefore favour the use of the
weight-weighted
form. However, the use of the length-
weighted average fibre length is preferred by Kajanni researchers who consider it to give a
better prediction of the paper making potential of the fibres. Accordmgly, in the current
study the length-weighted average fibre length is employed.
A.2.4 Handsheet preparation
Many papermaking tests require more than one sheet of paper, so it is convenient to
prepare, unde r identical conditions, several paper sheets from each pulp sample. These
pape r sheets, called handsheets, must be as uniform as poss ible. Tw o sheetm aking
77
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 195/220
systems are refered to in the ISO standards, conventional sheet former and the Rapid-
Kothem form er. Th e method to mak e handsheets on the conven tional sheet former is
introduced below.
The first step in preparing pulp for handsheets is to decide what beating points will be
used. Since beating changes the properties of pulp and paper, it is usual to make a set of
handsh eets from pulp beaten to at least four different levels, one unbeaten and three at
progressively higher beating revolutions. This
aUows
curves to be drawn which can be
used to interpolate paper properties.
Having dec ided on the am ount of beating, the next step is to disperse the specified quantity
of pulp thoroughly in water. This is usually achieved w ith a disintegrator of specified
design [ISO 5263-1979(E )].
The amount of pulp needed is determined by the grammage (mass per unit area) of the
handsheets and the number of sheets to be made. Typ ical handsheet gramm ages are
60.0±3.0 grams per square metre (calculated on an oven dry basis) for ISO standards
5269/1-1979 (E) using a conventional sheet former.
After disintegration, the
pulp is subjected to beating and measurem ent of the freeness. The
pulp is diluted with w ater to a stock concentration suitable for the preparation of
handsheets at tiie selected gramm age. This is convenientiy do ne using a stock divider,
which provides continuous agitation of the pulp suspension to maintain stock uniformity.
An appropriate volume of stock is taken from the stock divider to make the first
handsheet. The oven dried weight of this first handshee t can be used to adjust the amount
of water in the stock divider so tiiat later sheets have the desired gram mage. The details of
the method used for sheet preparation should refer to ISO 5269/1-1979 (E).
After tiie handsheets are prepared, they are dried under conditions designed to prevent
shrinkage. In the conventional sheetmaking equipm ent, this is achieved by attaching the
178
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 196/220
wet sheet to a rigid drying plate using a press, then allowing the sheet to air-dry in contact
with the plate.
A 2 5 Fibre strength
The strength of a fibre is fundamentally attributable to the fibril angle (the angle at which
the cellulose molecules spiral about the fibre axis) and the absence of weak spots in tiie
fibre due to exces sive breaks m the ceUulose mo lecular chain. Such cleavage is pre-
dominantiy a consequence of the pulping and bleaching processes employed to isolate and
purify the pulp fibre from the wood structure where it originates (see section 3.5.2). Suice
breaks in the molecular chain will reduce the average chain length, it foUows that the
viscosity test will monitor the extent of molecular chain breakage.
A standard viscosity test is conventionally used to measure pulp strength. Techn ically, it
measures no such thing, when carefully done, it provides a measure of the average chain
length of the cellulose molecules obtained by dissolving the pulp sample in a suitable
solvent, such as cupriethylenediamine solvent.
A zero-span tensile test on a sheet of paper measures the average strength of the fibres
which are carrying the tensile load when failure occurs. It thus depends on both the
number of fibres and their average strength and is a basic m easure of fibre quality.
Comparison of fibre quality, particular in a given mill environment, are generally made at
close to the same fibre conditions. W hen this is the case, the zero-span tensile test is a
comparative measure of fibre strength.
It is generally understood that the loss of fibre strength at high a-cellulose contents results
from the degradation of cellulose. Homogeneous degradation is random, causes little
strength loss and can be indicated by either zero-span tensile strength or pulp viscosity
value s. However, localized degradation in strength loss is unlikely to be recoverab le and
measurem ents of pulp viscosity may be misleading (G um agul, 1992). Thus care should
always be taken when interpreting the strength results.
179
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 197/220
A.2.5.1 Viscosity of pulp (captilary viscometer method)
TAPPI T230 om-89 viscosity of
pulp
method describes a procedure for determining the
viscosity of 0.5% cellulose solution, using 0.5M cupriethylenediamine as a solvent and a
capillary viscometer.
To measure pulp viscosity the general procedure includes:
(1) weigh 0.2500 g air-dried, moisture free pulp into dissolving bottle;
(2) add 25.00 ml distilled water and shake to disperse the pulp;
(3) add 25.00 ml cupriethylenediamine solution and purge with nitrogen for 1 min;
(4) cap the bottle and shake for few hours until the fibre is completely dissolved;
(5) conduct viscosity measurem ent, the viscometer size is selected to give efflux times of
over 100s, but less than 800s, and two specimens' efflux time should w ithin ±2s range.
(6) viscosity is calculate as V = Ctd
where V: viscosity of cupriethylenediamine solution at 25.0°C,
m-Pa-s(cp)
C: viscometer constant found by calibration
t: average efflux time , sec
d: density of the pulp so lution, g/cm- (=1.052)
A.2.5.2 Zero-span tensile test
The zero-span breaking length appears to be an index of the breakmg lengtii (or tensile
stiength) of a pulp beaten to its
maxhnum
value under ideal conditions. Consequentiy, it
is an excellent measure of tiie maximum strength of a pulp , and is almost completely
independent of the laboratory beating procedu re used. How ever, zero-span test as a dkect
measure of fibre strength suffers from the fact that changes in fibre length and / or
interfibre bonding influence the test value. A simple way to eliminate the influence of
interfibre bonding is to wet tiie sheet of paper prior to testing (wet zero-span te st). Fibre
length influence can be eliminated by adjusting the initial span to zero withou t any finite
value.
180
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 198/220
In our laboratory, wet zero-span tensile sU-ength is conducted on a Pulmac Zero-span
Tester (F ig. A.8). To do this, cut the handsheet into a 20 mm by 100 mm strip, wet the
strip and clam p between tw o jaw s (at zero span position). Then apply the tensile force to
fracture and calculate zero-span tensile strength (ZSTS, kN / m) and its index (ZSTI, Nm /
g) as foUows:
ZSTS = (P - Po) X C X 0.654
ZSTI = ZSTS X 1000 / Grammage (g/m')
W here: P is instrument reading, PQ is the conversion factor constant for zero opening and
C is the instrument factor, about 0.370.
Fig. A.8. Schematic illustration of Pulmac Zero-span tester.
A.3 Fab rication composite m ateria l
It has been reported in Chapter one that ceUulose fibre-cement sheets are commerciaUy
manufactiired on a Hatschek machine. In our smaU scale laboratory work the composite
materials are prepared by slurry / vacuum dewatering technique which emulates the
commercial Hatschek process. This technique includes slurrying the fibre-cement
formulation, vacuum dewatering, press and autoclaving (or air-curing).
181
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 199/220
A.3.1 Materials
The matrix was made from fresh commercial grade ordinary Portiand cement (OPC)
(Australian Cement, Geelong, Type A) and finely ground sihca (Steetiy brand lOOWQ).
The matrix grade and particle size might be important to material final properties,
how ever, such study is not included in this work. Considerable work in tiiis area would be
done by the manufacturing companies them selves.
During the manufacturing process or laboratory work, attention should be paid to storage
of the cement. At all stages up to the time of
use,
cement must be kept dry so as to prevent
or minimize deterioration from the effects of moisture, atmospheric humidity and
carbonation. In our experimental work, cement was purchased from local building
materials store, then batched and sealed tightiy mto several plastic jars. It is suggested that
for cement wh ich is old than four months should be classified as aged and re-tested
before use (Taylor, 1969).
Table A3 Natural fibre reinforced cement composites ingredients
Fibre % by
2
4
6
8
10
12
mass
water (ml)
300
350
400
500
500
500
28-day Air-curing
OPC
175°C, 7.5h Autoclaving
OPC:Silica
1:1
It is usually necessary to add other raw m aterials or special additives to the fibre cement
fumish / products, such as flocculating agents (processing aid), PFA (cheap
fiUer
/
pozzolanic), ball
clay
(interlaminate bond agent), microsilica (void filler / pozzolanic /
interlaminate bond agent), AH , (reduction m oisture movement). Hence the choice of one
or more of these special additives enables the designed properties of the end products to be
obtained. However, no additives were used in this thesis work.
82
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 200/220
The reinforcmg fibres were prepared by the method described in section A. 1. Wood fibre
P.radiata)
was supplied from Australia APM, Maryvale miU as specially selected high
tear wood chips and high tear Kraft pulped unbleached dry lap paper forms. Bamboo fibre
[Sinocalamus affinis Rendle) McC lue] was obtained fiom Kraft pulped unbleached
comm ercial packaging paper from Chang Jiang Paper
MUI
and Jian Xi Paper Mill, China.
A.3.2 Slurry / Vacuu m dewatering and press technique
Each samp le is based on a 130g dry weight of ingredients (Table A3). As the moistiire
content of the pulp is known, thus the appropriate weight of pulp fibre (oven dried base)
can be determined and then suspended in 300 to 500 ml of water (see Table A3) with
stirring for 2 m inutes. In some cases, weighed moist crumbled pulp was suspended in
much more water and disintegrated for a total of 12,500 revolutions to ensure fibre
dispersion, then dewatered to required am ount. The preweighed m atrix ingredients were
mixed thoroughly and then slowly added to the stirred suspension of fibres. Rapid stirring
took place for 5 minutes and the mixture was poured into an evacuable casting box (125
mm by 125 mm) with filter paper and fine wire screen (Fig. A.9) so that it could be
distributed over the screen. An initial vacuum was drawn until the sheet appeared dry on
the surface and then the sam ple was flattened carefuUy with a tamper. A vacuum of 60
KPa (gauge) was app lied for a total of 2 min. The sheet was then removed from the filter
screen. The sheet and screen were stored between two steel plates and the procedure
repeated until a stack of four sheets had been prepared. The stack of sheets was then
pressed for 10 minutes at a pressu re of 3.2 MPa . During the pressing extracted water was
blown away by compressed air and the load was applied and / or released slowly to
prevent any damage to the sheets.
183
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 201/220
filter paper / wire
Vacuum dew fc
7777777
Fig. A.9. Vacuum dewatering casting box
Coutts and Warden (1990) studied the influence of casting pressure and time on the
WFRC mechanical and physical properties for the slurry / vacuum dewatering technique.
They found that increasing the casting pressure and time resulted in improving
com pos ites' mechanical properties and increasing the density of the com posites. They
suggested that composites containing 8-10% fibre loadings can usually achieve
satisfactory properties when subjected to pressure for a short application time, whereas
high fibre loadings might need longer press time.
After pressing, the screens were carefully remov ed from the sheets which were then
stacked flat in a sealed plastic bag for 24 hours prior for cu ring.
A.3.3 Air curing and au toclaving
There are few cement curing models such as water curing, air-curing and autoclaving. Air-
curing and autoclaving are used for fibre reinforced cemen t produc ts. Cem ent composites
are cured under ambient conditions and
wiU
take 28 days to achieve their fuU strength
(samples remained in the plastic bag for 7 days then cured under the ambient conditions).
The air-cured method saves on capital investment in an autoclaving facility and has
superior properties than those of autoclaved prod ucts. Air-curing model is suitable for on-
site fabrication and for manufacturing polymer fibre reinforced cement products due to
mild curing conditions.
184
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 202/220
In both Australia and Europe, the manufacture of natural fibre reinforced cement
composites have been based on sheet products formed by techniques akin to the Hatschek
process and cured in high-pre ssure steam autoclaves. Steam at tempe ratures close to
180°C enables the replacement of between 40 to 60
of ordinary Portland cement by the
less expen sive siHca, which can reac t with the cem ent to form a calcium siHcate matrix of
acceptable strength (Lea, 1976). The reaction is completed within hours [ 125 psi
(180°C), 8 hours] instead of air-curing which takes weeks to achieve fuU strength. So the
storage facilities can be less as tumov er is faster.
The conditions for autoclaving were initiaUy studied by Coutts and Warden (1984b).
Composites autoclaved under 180°C, 125 psi steam, 8 hours at cement / sihca ratio T.l
demonstrated the best mechanical properties (see Fig. A. 10 and Al 1). At that condition,
fibre was not expected to be subjected to significant degradation while the composite
achieved adequate strength. However, there is a need to optimize the autoclaving
condition with different fibre sources, matrix ratios and fabrication process.
1:1,180*,8h(REF.
8)
1:1,170 ,4h
0 4 8 12
FIBRE CONTENT ( by mass
Fig.
A.IO.
Optimize autoclaving temperature and time
185
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 203/220
50:50
3 :67 38 :62 43 :57
0PC:Si02
RATIO (by mass)
Fig. A. 11. Optimize composite
OPC:Silica
matrix ratio for autoclaving
A.4 Cha racterisation of composite m aterials
Specimens were cut with a diamond saw to specified dimensions and stored under a
controlled atmosphere of 50±5% relative humidity and
22±2°C
for 7 days before testing.
Specimens measuring
125
mm by 40 mm (of varying thickness) were tested for flexural
strength and fracture tou ghness values. The flexural strength was measured as the
modulus of rupture (MOR) in three-point bending as: 3PL/2bd- where
P
is the
maximum load recorded during the test,
L
is the specimen span,
b
is the specimen width
and
d
is the specimen depth. A span of 100 mm and a deflection rate of 0.5 mm/min was
used on an Instron testing mach ine (Model 1114). The results of the tests were obtained
using automatic data coUecting and processing equipmen t. The fracture energy was
calculated from the area under the load / deflection curve w hen the faUed specimen
reached 50% maximum load. The fracture toughness is given by the fracture energy
divided by the cross-section area of the specimen. The comparison of fracture energy or
fracture toughness is strictiy only valid for specimens of the similar thickness.
186
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 204/220
Water absorption, density and apparent void volume physical properties were obtained
using the methods laid down in ASTM C948-81, which measures specimen dry weight,
wet weight and amount of water the specimen displaced.
In all cases, at least six specunens were tested for MOR, fracture toughness, density, void
volume and water absorption. 3 standard deviations have been included for
aU
properties
measured.
187
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 205/220
Appendix B :
Determination of Kraft Pulping Parameters
Consider a wood chip sample of X% moisture content. The requirement is to pulp A gm
equivalent oven dried chips at Y% active alkali and Z%o sulphidity at a liquor to wood ratio
ofR: l .
1.
Wt of air dried chips required = A/(100-X)*100 = B gms;
2. Grams of active alkaH required = A;=Y/100 = C gms;
3.
At Z% sulfidity, grams sulfide required = C ;<Z/100 = D gms;
4. Vol. of Na2S soln., = D*1000/ [Na2S] = E
mis;
where NajS is concentration of Na.S solution in g/1 (as
Na^O);
5. For NaOH in Na.S soln; gms NaOH in Na.S soln = E*[NaOH]71000 = F gms;
where
[NaOHJj
is concen tration of NaO H in
Na2S
solution in g/1 (as NajO);
6. Grams of NaO H reqd. = C-D-F = G gms;
7. If concentration of NaO H solution is [NaOH ];
then vol. NaOH =
G*
1000/[NaOH]; = H mis;
8. Total liquid
requked = A«R
=
1
mis;
9.
Mis
of
HjO
in wood chips
=
B-A = J mis;
10. Vol. of to HoO be added = I-(J-i-H+E) = K
mis.
Hence charge per bomb is:
1.
B gms of air dry chips equivalent to A gms oven dried;
2. H mis of [NaO HJi, of NaO H solution;
3.
E
mis
of
NajS
solution;
4. K mis of water.
188
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 206/220
References:
Ahn, W.Y . and Moslem i, A.A. (1980) SEM exam ination of wood-Portland cement
bonds.
Wood
Set
13,77-82.
Akers, S.A.S. and Garrett, G.G. (1983a) Observations and predictions of fracture in
asbestos-cement composites. J. Mater. Set. 18, 2209-2214.
Akers, S.A.S. and Garrett, G.G. (1983b) Fibre-matrix interface effects in asbestos-
cement com posites. J. Mater. Set. 18, 2200-2208.
Akers, S.A.S., Crawford, D.,Schultes, K., and Gerneka, D.A. (1989) Micromechanical
studies of fresh and weathered fibre cement composites.
Int. J. Cement Com p.
Lightweight Conor.
11 , 117-123. (and refs. therein).
Andonian, R., Mai, Y.M. and
CottereU,
B. (1979) Strength and fracture properties of
ceUulose fibre reinforced cement composites. Int. J. Cement C omp. Lightweight
Conor. 1, 151-158.
Anon. (1981) New - a wood fibre cement buildmg board. CSIRO Industrial Research
News
146, 1-2.
Arno, J.N., Frankle, W.E. and Sheridan, J.L. (1974) Zeta potential and its application
filler retention. Tappi 57(12), 97-100.
Assumpcao, R.M.V. (1992) UNIDO's programme in non-wood pulpmg and
papermaking.
2nd Int No-wood Pulping Papermaking C onf
Oct. 92, China, 5-19.
Atchison J.E. (1983) Data on non-wood plant fibres. Pulp and Paper Manufacture
Vol.1 Properties of fibrous raw materials and their preparation for pulping.
(Ed. Kocurek, M .J. and Stevens, C.F.B.), TAPP I, U.S.A. 1983.
Australian Patent (1981) No. 515151, issued March 19tii, 1981,
James Hardie and Co.
Pty Ltd.
Australian Patent (1982) No. 554185,
James Hardie and C o. Pty L td.
189
to
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 207/220
Aveston, J., Cooper, G.A. and KeUy, A. (1971) The properties of fibre composites.
Conf.
Proc. National Physical Lab. London, Nov. 1971, P15.
Barnes, B.D ., Diamond , S. and Dolch, W.L. (1978) The contact zone between Portiand
cement paste and glass aggregate surfaces. Cem. Conor. Res. 8, 233-243 .
Bazant, Z.P . (1985) M echanics of fracture and progressive cracking in concrete structures.
Fracture M echanics of Concrete: Structural Application and Numerical
Calculation,
1-5.
Bentur, A., Mindess, S. and Diamond, S. (1985a) PuU-out processes m steel fibre
reinforced cement.
Int
J. Cement Comp. Lightweight Conor. 7 (1), 29-37.
Bentur, A., Diamo nd, S. and M indess, S. (1985b) The
microsh-ucture
of the steel fibre-
cement interface.
/ .
Mater.
Set
20, 3610-3620.
Bentur, A. and Mindess, S. (1990) Fibre R einforced C ementitious C omposites,
Elsevier Science PubHshers Ltd., 99-109.
Brown, R.B. (1932) Paper Trade J. 95, 145.
CampbeU, M.D. and Coutts, R.S.P. (1980) Wood fibre-reinforced cement composites.
/ . Mater. Set 15, 1962-1970.
Clark,
J.d'A.
(1962) Effect of fibre coarseness and length. 1.
Bulk,
burst, tear, fold and
tensile tests. Tappi 45(8), 628-634.
Clark, J.d'A. (1985)
Pulp Techn ology and Treatment for paper.
MiUer Freeman,
2nd Ed. Chap. 17.
CoUey, J.
1913)Appita
27(1), 35
Coutts, R.S.P. (1979a) Wood fibre reinforced ceme nt com posites. CSIRO Division of
Chemical Technology. Research Review 1979.
Coutts, R.S.P. and CampbeU, M.D. (1979b) CoupHng agents in wood fibre-reinforced
cement composites. Com posites 10, 228-232.
190
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 208/220
Coutts, R.S.P. and Ridik as, V. (1982a) Refmed wood fibre cement products. Appita
35(5),
395-400.
Coutts, R.S .P. and Kigh tiy, P. (1982b) M icrostructure of autoclaved refmed wood-fibre
cement mortars. J. Mater. Sci 17, 1801-1806.
Coutts, R.S.P. and M icheU, A.J. (1983a) Wood pulp fibre-cement composites. / . ppl
Poly. Set: Appl. Poly. Symp. 37, 829-844.
Coutts, R.S.P. (1983b) Wo od fibre in inorganic matrices. Ch em. in A ustralia
50, 143-148.
Coutts, R.S.P. (1983c) Flax fibres as a reinforcement in cement mortars. Int. J. Cement
Comp. Lightweight Conor. 5(4) 257-262.
Coutts, R.S.P. (1984a) Autoclaved beaten wood fibre reinforced cement composites.
Composite 15(2), 139-143.
Coutts, R.S.P. and Kightiy, P. (1984b) Bonding in wood fibre-cement composites.
J. Mater. Sci. 19, 3355-3359.
Coutts, R.S.P. and Warden, P.G. (1985) Air-cured wood pulp fibre cement composites.
/ Mater. Sci. Lett. A, 117-119.
Coutts, R.S.P. (1986) High yield wood pulps as reinforcement for cement products.
Appita 39(1), 31-35.
Coutts, R.S.P. (1987a) Eucalyptus wood fibre reinforced cement.
/ . Mater. Sci. Lett.
6, 955-951.
Coutts, R.S.P. and Warden, P.G (1987b) Air-cured abaca reinforced cement composites.
Int. J. Cem ent Comp. Lightweight Conor. 9(2), 69-73.
Coutts, R.S.P. and Ward, J.V. (1987c) Microstructure of wood fibre plaster composites.
J. Mater. Sci. Lett.
6,
562-564.
Coutts, R.S.P. (1987d) Fibre-matrix mterface m air-cured, wood pulp, fibre cement
191
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 209/220
composites. / . Mater. Sci. Lett. 6, 140-142.
Coutts, R.S.P. (1987e) SEM study of high yield wood pulps in cement matrices.
unpublished results.
Coutts, R.S .P. (1988) W ood fibre reinforced ceme nt composites. Natural Fibre
Reinforced Cement and Concrete V ol. 5 (Ed. R.N. S wamy), Conor. Tech. &
Design, Blackie, London, 1-62.
Coutts, R.S .P. (1989a) Wastepaper fibres m cem ent products. Int. J. Cement Comp.
Lightweight Conor. 11(3), 143-147.
Coutts, R.S.P . and W arden, P.G. (1989b) unpubHshed results.
Cou tts, R.S .P. and W arden, P.G . (1990a) Effect of compaction on the properties of
air-cured wood fibre reinforced cement. Cement
Cone. Composites. 12, 151-156.
Coutts, R.S.P . (1990b) Banana fibres as reinforcement for building products.
/ . Mater. Sci. Lett. 9, 1235-1236.
Coutts, R.S.P. and Warden, P.G. (1992a) Sisal pulp reinforced cement mortar.
Cement Cone. Composites. 14,17-21
Coutts, R.S.P.(1992b) From forest to factory to fabrication. Fibre Reinforced Cem
Conor RILEM 92,
p31-47
(eds R.N.Sw amy), E & FN Spon, London.
Coutts, R.S.P. and Ni, Y. (1994a) Autoclaved bamboo pulp fibre reinforced cement.
Cement Cone. Co mposites, in press
Coutts, R.S.P., Ni, Y., and Tobias, B.C. (1994b) Air-cured bamboo pulp reinforced
cement. / . Mater. Sci. Lett. 13, 283-285.
Cote, W.A .
(1980)
Papermaking Fibres, a Photomicographic Atlas. Syracuse Univ.
Press, NY.
Crabtree, J.D. (1986) Past, present and future developments in mdustrial fibre cement
technology. K eynote Speech, 2nd Int. RILE M Sym p. Sheffield, 15 July.
192
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 210/220
Davies, G.W., CampbeU, M.D. and Coutts, R.S.P. (1981) An SEM study of wood fibre
reinforced cement composites. Holzforschung 35, 201-204.
Fordos, Z. and Tram, B. (1986) Natural fibre as remforcement in cement based
composites. RILEM FRC86 (eds R.N.Swamy, R.L.Wagstaffe and D.R.Oakley),
RILEM Technical Committee 49-TFR, Paper2.9.
Gale, D.M., Shah, A.H. and Balaguru, P. (1990) Oriented polyethylene fibrous pulp
reinforced cement comp osites. Thin-section Fibre Reinforced C oncrete and
Ferrocement.
American Concrete Institute Publication SP 124-4, 61-77.
Giertz, H.W . (1961) Effect of pulping processes on fibre properties and paper structure.
Proceedings
of Cambridge / Oxford Symposia. Mech. Engineering, London.
Gordon, J.E. (1976) T he New Science of Strong M aterials - or W hy You Don't Fall
Through the Floor.
2nd edn.. Penguin Book s, 287pp.
Gordon, J.E. and Jeronimidis, G. (1980) Composites with high work of fracture.
Phil Trans. R. Soc. London Ser. A., 294, 545-550.
Gram, H-E. (1983)
Durability of natural fibres in concrete.
Swedish Cement and
Concrete Research Institute, Stockholm.
Gum agul, N.; Page, D.H. and Paice, M.G . (1992) The effect of ceUulose degradation
on the strength of wood pulp fibres. Nordic Pulp and Paper Research Journal
3,
152-154.
Hannant, D.J. (1978) Fibre Cements and Fibre Concretes, John Wiley, Chichester,
1-50, 61-134.
Harper, S. (1982) Developmg asbestos free calcium silicate building boards. Composites
A,
123-128.
Hibbent, A.P. and Hannant, D.J. (1982) Toughness of fibre cement composites.
Composites 13 , 105-111.
193
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 211/220
Higgins, H .G., Gariand, C.P. and Puri, V. (1977) Thermomechanical and
chemitiiemiomechanical pulps from eucalypts and other hardwoods. Appita, 30(5),
415-423.
Higgins, H.G., Puri, V. and Gariand, C.P. (1978) The effect of chemical pretreatments
in chip refining. Appita, 32(3), 187-200.
Hodgson, A.A. (1985) Alternatives to Asbestos and Asbestos Products.
Anjalena Publications Ltd. UK, 7 1-85, 150-158.
Hughes, D.C. and Hannant, D.J. (1985) Reinforcement of Griffith flaws in cellulose
reinforced cemen t composites.
/
Mater. Sci. Lett. 4, 101-102.
James Hardie Industries (1984) The name behind the names 71pp.
James Hard ie and Co. Pty Ltd. Report on paper pulp sheets. Internal Report Nov. 1947.
Jayne, B.A. (1959) Mechanical properties of wood fibres. Tappi 42 (6), 461-467.
Johnston, C D . (1975 ) Steel-fibre-reinforced mo rtar and concrete: A review of
mechanical properties. Fibre reinforced Concrete. American Concrete Institute
Publication S.P.44, 127-142.
Kerekes, R.J. and Tam D oo,
P.A.
(1985) Wet fibre flexibility of some major softwood
species pulped by various processes.
/.
Pulp and Paper Sci.
11(2),
J60-J61.
Kibblewhite, R.P. (1989) Effects of pulp drying and refining on softwood fibre wall
structural organisations. Funda mentals of Paperm aking, Vol 1.
Trans,
of the
Cambridge Symp.
(Ed. C.F. Baker & V.W . Punton), Mech. Engineering
Publication. Ltd., London, 121-152.
Kim, C.Y., Page, D.H.,
El-Hosseiny,
F. and Lancaster, P.J. (1975) Mechanical properties
of single wood pulp fibres.
III.
Effect of drying sttess on stiength. Appl.
Polym.
Sci.
19(6), 1594-1562.
Kuang, S. J., Zhang J. and Zhang, R.L. (1992) A study on the Characteristics of
194
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 212/220
blended fumishes of wheat straw pulp and long fibre pulps.
China Pulp Paper
11(2), 3-10.
Kurdin, J.A. (1983) The pulp of the twenty-first century.
Tappi
66(6), 9-19.
Laws, V. (1983) On tiie mixture rale for strength of fibre reinforced cement. /. Mater.
Sci. Lett. 2, 527-531.
Lea, F.M. (1976)
The Chemistry of Cement and Concrete.
3rd edn., Edward Arnold,
London.
Lhoneux, B. de and Avella, T. (1992) Fibre-matrix interactions in autoclaved ceUulose
cement composites.
RILEM FRC92
(ed. R. N. Swamy), published by
E & F N Spon, London, 1152-1165.
Lhoneux, B. de; Avella, T. and Garves, K. (1991) Influence of autoclaving on chemical
pulp fibre properties for fibre-cement applications. Holzforschung. 45(1), 55-60.
Li,
W .L. (1992) Major R & D achievements of the paper industrial research institute
(PIRIC)
in the past years (1980-1990). China Pulp Paper 6(3), 56-60.
Lola, CR. (1986) Fibre reinforced concrete roofing technology appraisal report.
RILEM FRC86
(eds R.N.Swamy, R.L.Wagstaffe and D.R.Oakley),
RILEM Technical Committee 49-TFR, Paper2.12.
Mai, Y.W. (1978) Strength and fracture properties of asbestos-cement mortar composites.
Univ. of Sydney. Technical Note S-13.
M ai, Y. W., A ndonian , R. and CottereU, B . (1980) On polypropylene-cellulose fibre-
cement hybrid composite. In Advances in Composite Materials, Paris, 1687-1699.
M ai, Y.W ., Hakeem , M.I. and CottereU, B. (1983) Effects of water and bleaching on the
mechanical properties of cellulose fibre cements.
J.
Mater.
Sci.
18, 2156-2162.
Mai, Y.W. and Hakeem, M.I. (1984a) Slow crack growth in bleached cellulose fibre
cements.
/ .
Mater. Sci. Lett. 3, 127-130.
195
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 213/220
Mai, Y.W. and Hakeem, M.I. (1984b) Slow crack growth in cellulose fibre cements.
J. Mater. Sci. 19, 501-508.
Martson, T.U., Atkins, A.G . and Felbeck, D.J. (1974)
/ .
Mater. Sci. 9,
AAl
M cKenzie, A.W . (1978) The structure and properties of paper.
Part XX.
The tensUe
properties of paper and papermaking fibres.
Appita
32, 207-212.
M cKenzie, A.W . (1985) Studies on soda-anthraquinone pulpuig. Part 1, Fibre damage
and tearing resistance. Appita. 38(6), 428-431.
McK enzie, A.W. (1992) Com monly asked questions on pulping and papermaking.
CSIRO Australia, Forestry and Forest Products Newsletter, 7, 1-3
MicheU, A.J. and Freischmidt, G. (1990) Effect of fibre curl on the properties of wood
pulp fibre-cement and silica sheets. /. Mater. Sci. 25, 5225-5230.
M indess, S. (19 ) The application of fracture m echanics to cemen t and concrete:
a historical review.
Fracture Mechanics of Concrete,
(ed. F.H. Wittman), Elsevier,
Amsterdam, 1-30.
M indess, S. and Bentur, A . (1982) Technical N otes: The fracture of wood fibre einforced
cement. Int. J. Cement Comp. Lightweight Conor. A, 245-249.
Morrissey, F.E., Coutts, R.S.P. and Grossman, P.U.A. (1985) Bond between ceUulose
fibres and cement. Int. J. Cement Com p. Lightweight Conor. 7(2), 73-80.
Mukherjee, P.S., Satyanarayana, K.G. (1986) An empirical evaluation of stmcture-
property relationships in natural fibres and their fracture behaviour. / .
Material Sci.
21,4162-4168.
NeveU, T.P. and Z eronian, S.H. (1984)
Cellulose C hemistry and its Applications.
EUis Horwood, Chichester, 506-530.
NeviUe, A.M. (1959) Some aspects of the stiength of concrete. Civil Eng., London, 54,
1153-1156
196
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 214/220
Nilsson, L. (1975) Reinforcement of concrete with sisal and other fibrous plaster
sheets.
Swedish Council for Building Research Document
D14.
PaavUainen, L. (1993) Importance of cross-dimensional fibre properties and coarseness
for the characterisation of softwood sulphate pulp. Paper and Timber.
75(5), 343-351.
PaavUainen, L. (1993) Conformability - flexibility and coUapsibiHty - of sulphate pulp
fibres. Paper and Timber. 75 (9-10), 689-699.
Pavithram, C , Mukherjee, P.S., Brahmakum ar, M. and Damodaran, A.D. (1987) Impact
properties of natural fibre composites. J. Mater. Sci. Lett. 6, 882-884.
Page, D.H. (1969) A tiieory for the tensUe strength of paper. Tappi 52(4), 674-681.
Page, D.H. (1970) The physics and chemistry of wood pulp fibres. Tappi 42 (6), 461-467.
Page, D.H., El-Hosseiny, F. and Winkler, K. (1971) Behaviour of single wood fibres
under axial tensile strain. Nature (London), 229, 252-253.
Page, D.H.; Seth, R.S. and EI-Hosseiny, F. (1985) Strength and Chemical
Com position of wood pump fibres.
Paperm aking Raw M aterials,
Trans,
of the
Oxford Sym p.
(Ed. V.W. Punton), Mech. Engineering PubHcation. Ltd., London,
77-91.
Page, D.H.; Seth, R.S.; Jordan, B.D. and Barbe, M .C (1985) Curi, crimps, kinks and
microcompressions in pulp fibres - their origin, measurement and significance.
Papermaking Raw M aterials,
Trans,
of the Oxford Symp. (Ed. V.W . Punton),
Mech. Engineering Publication. Ltd., London, 183-227.
Pakotiprapha, B ., Pama, R.P. and Lee, S.L. (1983) B ehaviour of a bamboo
fibre-cement paste composite. J.
Ferrocement
13, 235-248 .
Pedersen, N. (1980) Commercial development of alternatives to asbestos sheet products
based on short fibres. Proc. Symp. on fibrous C oncrete London, 16 AprU 1980,
197
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 215/220
189-193.
Pinchm, D .J. and Tabor, D. (1978) hiterfacial phenomena in steel fibre reinforced
cement. I: Structure and strengtii of the interfacial region. Cem. C onor. Res.
8(1),
15-24.
Romualdi, J.P. and Batson, B.V. (1963) Fibre reinforced concrete properties.
/ . Eng. Mechanics division, ASCE, 89(EM3), 147-168.
Romualdi, J.P. and Mandel, J.A. (1964) /. Am. Concrete Inst. Detroit, 61, p657.
Seth, R.A. and Page, D.H. (1987) Fibre properties and tearing resistance.
Proceed ings of 1987 Int. Pap er Physic Conf Auberge Mont Gabriel, Quebec.
9-16.
Seth, R.S . (1990) Fibre quality factors in papermak ing - 1. the importance of fibre
length and strength. Mat. Res. Soc. Symp. Proc. Vol.197, 125-141.
Seth, R.A., Francis, D.W. and B ennington, C.P.J. (1991) The effect of mechanical
treatment during medium consistency fluidisation on pulp properties.
TAPP I proceedings of 1991 Pulping C onf 713-722.
Seth, R.A., Francis, D.W. and Benn ington, C.P.J. (1993) The effect of mechanical
treatment during medium consistency fluidisation on pulp properties.
Appita. 46(1), 54-60.
Sharman, W.R. and Vautier, P.B . (1986) Durability studies on wood fibre reinforced
cement sheet. RILEM FRC86 (eds R.N.Swamy, R.L.Wagstaffe and D.R.Oakley),
RILEM Technical Committee 49-TFR, Paper2.12.
Simatupang, M.H. and Lange, H. (1987) Lingnocellulosic and plastic fibres for
manufacturing of fibre cement boards. Int. J. Cement C omp. Lightweight Conor.
9(2), 109-112.
Sinha, U.N., Dutta, S.N., Chaliha, B.P. and Iyengar, M.S. (1975) PossibiHties of
198
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 216/220
replacing asbestos in asbestos cement sheets by cellulose pulp.
Indian Conor. J.
8, 228-232.
Singh, S.M. (1979) Investigations into the causes of poor strength of Portland cement
bonded
lignocellulosic
m aterials, /.
Ind. Acad.
Wood Sci. 10(1), 15-19.
Singh, B. and Majumdar, A.J. (1981) Properties of GRC containing inorganic fillers.
Int. J. Cement Comp. Lightweight Conor. 3(2), 93-102.
Smook, G.A. (1989)
Hand book for Pulp and Paper Technologists.
TA PPI, U.S.A.
7th printing. Chap. 7 and 22.
Staff of the Institute of Paper Chemistry (1944) Paper Tr. J. 118(5), 13.
Stevens, M.G. and CantriU, E.R. (1992) Influence of autoclaving on Kraft w ood
pulp
cellulose determined by differential thermal analysis. Materials Forum.l6, 359-363.
Stone, J.E. and Clayton D.W. (1960) The role of sulphide in the Kraft process.
Pulp and Paper Magazine of Canada, June, T307-313.
StreHs, I. and Kennedy , R.W . (1967) Identification of North American Commercial
Pulpwoods and Pulp Fibres. Univ of Toronto Press, Canada.
Stucke, M .S. and Majumdar, A.J. (1976) Microstructure of glass fibre-reinforced cement
composi tes . / .
Mater. Sci.
11 ,
1019-1030.
Studinka, J.B. (1989) Asbestos substitution in the fibre cement industry.
Int. J. Cement Com p. Lightweight Conor. 11(2), 73-78.
Swamy, R.N. and Mangat, P.S . (1974) Proc. Inst. CivilEngrs., 57, p70 1.
Swift, D.G. and Smitii, R.B.L. (1979) The flexural strength of cement-based composites
using low modulus (sisal) fibres. Composites 10,145-148.
Tam Doo, P.A. and Kerekes, R.J. (1982) The flexibility of wet pulp fibres.
Pulp and Paper Canada 83(2), 46-50.
Taylor, W.H. (1967)
Concrete Technology and Practice.
Angus and Robertson,
199
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 217/220
Sydney, 3rd Ed. Chap 3.
Teder, A. and 01m, L. (1991) Proceedings Inter. Confer, on Bleached Kraft Pulp
Mills,
4-7th Feb. 1991, Melboume, 59-80.
Thomas, N.L. and BirchaU, J.D. (1983) The retardmg action of sugars on cement
hydration. Cem . Con or. Res. 13, 830-842.
US Patent (1884) N o. 291114, 1 Jan. 1884
US Patent (1899) No. 635221, 17, Oct. 1899
US Patent (1900) No. 658590, 25, Sept. 1900
Vinson, K.D. and Daniel, J.I. (1990) Specialty ceUulose fibres for cement
reinforcement.
Thin-section Fibre Reinforced Concrete and Ferrocement.
American Concrete Institute Publication SP-124, 99-124.
Wai, N.N., Nanko, H. and Murakami, K. (1985) A morphological study on the
behaviour of bamboo pulp fibres in the beating process. Wood Sci. Technol.
19,211-222.
Wang, J.H., Guo, X.P., Xue, C Y . and Wang , R. (1993) The Ultrastmcture of bamboo
fibres and its behavior during beating. China Pulp and Paper 8(4), 10-17.
Walton, P.L. and Majumdar, A.J. (1975) Cement-based composites with mixtures of
different types of fibres. C omposites, 9, 209-216.
Watson, A.J., Wardrop, A.B.,
DadsweU,
H.E. and Cohen, W .E. (1952) Aust. Pulp
Paper Ind. Tech. Assoc. Proc. 6, 243.
Watson, A.J. and Hodder, I.G. (1954)
A ust. Pulp Pap er Ind. Tech. Assoc. Proc.
8, 290.
Watson, A.J. and DadsweU, H.E. (1961) Influence of fibre morphology on paper
properties, part 1. fibre length.
A/;/;iYa
14(5), 168-178.
WilHams, M.F. (1994) Matching wood fibre characteristics to pulp and paper processes
2
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 218/220
and products. Tappi Journal, 77(3), 227-233.
Zhao, J., Zheng , G.M., W ang, H.D. and Xu, J.L. (1990) The Natural History of China.
CoUuis
Publication Ltd. US A, 62-65.
201
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 219/220
Bibliography:
Balodis, V. (1991) Notes on pulpwood quality asse ssme nt CSIRO Div. Forest
Products, Australia.
Bentiey, R.G., Scudamore, P. and Jack, J.S. (1994) A comparison between fibre length
measurement methods. Pulp and Paper C anada, 95(4) 41-45.
Coutts, R.S.P. (1980)
W ood fibre cement comp osites prepared by slurry/vacuum
box techniques. CSIR O D iv. Chemical Technology. Program report, CB 203.
Irvine, G.M. (1992) M onash U niv. M.Eng.Sc. pulp and paper laboratory course
Topic. 8.3 chemical pulping. CSIRO D iv. Forest Products.
Mai, Y.W. (1978) Strength and fracture properties of asbestos-cement mortar composites.
Univ. of Sydney. Technical Note S-13.
McKenzie, A.W. (1989) The tear-tensile relationship in softwood pulps.
Appita
42(3),
215-221.
McKenzie, A., McHenry, M. and McKenzie, M. (1991)
Monash Univ. M.Eng.Sc.
pulp and pap er laboratory course Topic.8.6 paper testing.
C SIRO Div. Forest
Products.
McKenzie, M. and McHenry, M. (1993) Pulp evaluation lab. manual part 1:
preparation of handsheets.
CSIRO D iv. Forest Products. FP 267
McKenzie, A.W. (1994)
A Guide to Pulp Evaluation.
Division of Forest Products,
CSIRO Austraha.
Nelson, P.J., Chin, W.J. and Grover, S.G. (1993) A study of ways of increasing the
use of non-chlorine-containing bleaching chemicals, increasing the effectiveness
of chlorine d ioxide, and improving the overaU efficiency of mo dem bleaching
sequences, part A. National pulp mills research progr am internal report. N o.l .
Pulmac
Instraments hiternational,
Canada. Fiber quality and fibre strength.
2 2
8/10/2019 Cement composites
http://slidepdf.com/reader/full/cement-composites 220/220
Technology series.
TAPP I Test Methods (1994-1995).
TAPPI Press, U.S.A.
Williams, W.D. (1992) M onash U niv. M.Eng.Sc. pulp and paper laboratory course
Top ic.8.2 pulping, part B. high-yield pulping. CSIRO Div. Forest Products,
FP 204.